refactor(lang): rename the str type (and stringify builtin) to string

Expand the abbreviated string type and its conversion builtin to the full
word everywhere:
  str            ->  string        (the immutable-string type)
  str(x) -> str  ->  string(x) -> string   (the stringify builtin;
                                            what `{…}` interpolation calls)

Types are recognized by identifier, and llty maps both spellings to LLVM
`ptr`, so this is an atomic source rewrite: type annotations, the Ludic
type tags, the builtin name/dispatch, and the interpolation desugar, plus
the grammars, LSP, docs (type-str -> type-string, fn-str -> fn-string), and
inventory. int/bool stay (universally accepted, like Math).

Reseeded; C-free fixpoint holds; all suites green (45/24/29); site + check.py OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

runtime/native/core.ludic

@@ -29,7 +29,7 @@ var rt_alive: int = 1              # platform still running?
 
 # 5x7 glyphs for ASCII 32..90, 7 rows per glyph, each row a 5-bit mask stored
 # biased by '0' so the whole font is one printable string literal.
-function rt_font() -> str {
+function rt_font() -> string {
   return "00000004444404000000000000000000000IJ4:FC000000000000000000000000000000E>O>E0044O4400000448000O000000004012448@@>ACEIA>4<4444>>A168@ON11>11N26:BO22O@N11A>>@@NAA>O124888>AA>AA>>AA?11>04004000000000024842000O0O000842480>A164040000000>AAOAAANAANAAN>A@@@A>LBAAABLO@@L@@OO@@L@@@>A@GAA>AAAOAAA>44444>7222BB<ABDHDBA@@@@@@OAKEEAAAAIEECAA>AAAAA>NAAN@@@>AAAEB=NAANDBA>A@>1A>O444444AAAAAA>AAAAA:4AAAEEKAAA:4:AAAA:4444O1248@O"
 }
 
@@ -203,7 +203,7 @@ function rt_glyph(x: int, y: int, ch: int, colour: int, sc: int) -> void {
   }
 }
 
-function rt_text(x: int, y: int, s: str, colour: int, sc: int) -> void {
+function rt_text(x: int, y: int, s: string, colour: int, sc: int) -> void {
   var i = 0
   var cx = x
   var ch = s[0]
@@ -329,7 +329,7 @@ function rt_running() -> bool {
 }
 
 # ---- writing the frame out ------------------------------------------------
-function rt_put_str(buf: ptr, at: int, s: str) -> int {
+function rt_put_str(buf: ptr, at: int, s: string) -> int {
   var i = 0
   var n = at
   var ch = s[0]
@@ -367,7 +367,7 @@ function rt_put_int(buf: ptr, at: int, v: int) -> int {
   return n
 }
 
-function rt_dump_ppm(path: str) -> void {
+function rt_dump_ppm(path: string) -> void {
   let f = file_open(path, "wb")
   if (f == null) { return }
 
@@ -412,7 +412,7 @@ function rt_map_size(w: int, h: int) -> void {
   fill(rt_map, 32, 96 * 64)
 }
 
-function rt_map_row(y: int, s: str) -> void {
+function rt_map_row(y: int, s: string) -> void {
   if y < 0 { return }
   if y >= 64 { return }
   var x = 0
@@ -438,7 +438,7 @@ function rt_tile(x: int, y: int) -> int {
 # so it survives whatever the caller does with the original.
 var rt_statusbuf: ptr = null
 
-function rt_status(s: str) -> void {
+function rt_status(s: string) -> void {
   var i = 0
   var ch = s[0]
   while ch != 0 {

runtime/native/image.ludic

@@ -42,7 +42,7 @@ function png_tag(b: ptr, at: int, a: int, c: int, d: int, e: int) -> bool {
   return true
 }
 
-function rt_read_file(path: str) -> ptr {
+function rt_read_file(path: string) -> ptr {
   let f = file_open(path, "rb")
   if (f == null) { return null }
   file_seek(f, 0, 2)
@@ -62,7 +62,7 @@ var png_w:  int = 0
 var png_h:  int = 0
 var png_px: words = null
 
-function rt_decode_png(path: str) -> bool {
+function rt_decode_png(path: string) -> bool {
   png_w = 0
   png_h = 0
   png_px = null
@@ -227,7 +227,7 @@ function rt_decode_png(path: str) -> bool {
 }
 
 # ---- images ---------------------------------------------------------------
-function rt_image_load(path: str) -> int {
+function rt_image_load(path: string) -> int {
   if img_n >= IMG_MAX { return 0 - 1 }
   if rt_decode_png(path) == false { return 0 - 1 }
   let id = img_n
@@ -315,7 +315,7 @@ function rt_draw_9slice(id: int, dx: int, dy: int, dw: int, dh: int, inset: int)
 }
 
 # ---- sprites (16x16 art) --------------------------------------------------
-function rt_png_load(path: str) -> int {
+function rt_png_load(path: string) -> int {
   if spr_n >= SPR_MAX { return 0 - 1 }
   if rt_decode_png(path) == false { return 0 - 1 }
   let id = spr_n
@@ -375,7 +375,7 @@ function rt_hexval(c: int) -> int {
   return 0 - 1
 }
 
-function rt_sprites_load(path: str) -> void {
+function rt_sprites_load(path: string) -> void {
   let d = rt_read_file(path)
   if (d == null) { spr_n = 0; return }
   let size = rt_file_len

runtime/native/truetype.ludic

@@ -166,7 +166,7 @@ function tt_pick_cmap(id: int, d: ptr, co: int) -> void {
   tt_cfmt[id] = tt_u16(d, best)
 }
 
-function rt_font_load(path: str) -> int {
+function rt_font_load(path: string) -> int {
   if tt_n >= TT_MAX { return 0 - 1 }
   let d = rt_read_file(path)
   if (d == null) { return 0 - 1 }

runtime/native/truetype_raster.ludic

@@ -269,7 +269,7 @@ function tt_glyph_get(font: int, cp: int, px: int) -> int {
 # ---- UTF-8 ----------------------------------------------------------------
 var u8_next: int = 0        # byte index just past the codepoint last decoded
 
-function tt_utf8(s: str, at: int) -> int {
+function tt_utf8(s: string, at: int) -> int {
   let c = s[at]
   var extra = 0 - 1
   if c < 128 { extra = 0 }
@@ -327,7 +327,7 @@ function tt_blit(bmp: ptr, w: int, h: int, dx: int, dy: int, colour: int) -> voi
   }
 }
 
-function rt_text_ttf(font: int, x: int, y: int, s: str, colour: int, px: int) -> void {
+function rt_text_ttf(font: int, x: int, y: int, s: string, colour: int, px: int) -> void {
   if font < 0 { return }
   if font >= tt_n { return }
   if px <= 0 { return }
@@ -351,7 +351,7 @@ function rt_text_ttf(font: int, x: int, y: int, s: str, colour: int, px: int) ->
   }
 }
 
-function rt_text_w(font: int, s: str, px: int) -> int {
+function rt_text_w(font: int, s: string, px: int) -> int {
   if font < 0 { return 0 }
   if font >= tt_n { return 0 }
   if px <= 0 { return 0 }

runtime/native/ui.ludic

@@ -168,7 +168,7 @@ function rt_ui_set(i: int, k: int, v: int) -> void {
   if k == K_SKIN { ui_skin[i] = v }
 }
 
-function rt_ui_static_text(i: int, s: str) -> void {
+function rt_ui_static_text(i: int, s: string) -> void {
   if i < 0 { return }
   if i >= UI_MAX { return }
   ui_text[i] = s

runtime/native/ui_draw.ludic

@@ -214,7 +214,7 @@ function rt_ui_clicked(id: int) -> bool {
   return ui_fired[id] == 1
 }
 
-function rt_ui_set_text(id: int, s: str) -> void {
+function rt_ui_set_text(id: int, s: string) -> void {
   if id < 0 { return }
   if id >= ui_n { return }
   let base = id * 96