feat(lang): 27.3 - a reducer on a table's row: reducer T in S.table on A, @Target, Row<T>, @RowVerb, @Column

An action names one row of a ludic.base Table<T> by its handle, in a field marked @Target, and
`reducer Deer in Herd.deer on Spook(r: mut Row<Deer>, n: Noise, a: Spook)` runs once, for that row
alone (the table may sit down a path, S.w.tab). The drain resolves the handle (tb_row) and hands the
reducer a Row<T> - new in ludic.base: tb, row, h, rec - that the queue keeps, one per row reducer,
filled in place, so a targeted action allocates nothing; a stale handle runs nothing, and
LUDIC_ACTIONS_LOG=1 prints a line for it (@alloc_ok). Row reducers order among an action's by their
state's name, then the table's path.

Checked at compile time (actions_rows.ludic): the row reaches r.rec and r.h only - r.tb / r.row
refused, the view never assigned, stored, copied or handed on except to a @RowVerb (a function of
the record's own module taking Row<T> first; any other function taking a row is refused); a field
marked @Column (a table column mirrors it) is not written through r.rec; only the module owning the
state declares a row reducer; one @Target, an int, per action; the states between the row and the
action are read. `mut` is allowed on a Row<T> parameter.

ludic schema's code section gains row_reducers (record, table, state, action, target, predicted,
net, module, at) and row_verbs (name, record, module, at), and every action its target; row
reducers are left out of `reducers`. ludic deps and ludic-lsp name a row reducer
`reducer Deer in Herd.deer on Spook`. vocab: @Target, @Column, @RowVerb; docs/language pages;
LANGUAGE.md "A reducer on a row"; examples actions/rows and ten rejects; test.ludic feat, reject and
schema cases (not run); changes/row-reducers.md. Reseeded; bootstrap-cfree fixpoint holds (307497
lines); Maroon Lake's `ludic build --check` is clean against this tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/check/check_state.ludic

@@ -108,11 +108,12 @@ function ck_write_check(t: Node, what: pointer) -> void {
     ck_err("state", t, `{what}: {name} is module-level and immutable all the way down; changing data belongs in a state, passed as a mut parameter`)
   }
 }
-# a function's parameters: `mut` is for a state, and a state without it is read-only
+# a function's parameters: `mut` is for a state (or a table's row, 27.3), and a state without it is
+# read-only
 function ck_bind_param(p: Node) -> void {
   ck_bind(p.s, p.ty)
-  if p.uns == 1 and not is_state_ty(p.ty) {
-    ck_err("state", p, `{p.s}: mut {p.ty} - mut is for a state parameter, and {p.ty} is not a state`)
+  if p.uns == 1 and not is_state_ty(p.ty) and not rr_is_row_ty(p.ty) {
+    ck_err("state", p, `{p.s}: mut {p.ty} - mut is for a state parameter (or a row, mut Row<T>), and {p.ty} is not a state`)
   }
   ck_set_ro(is_state_ty(p.ty) and p.uns == 0)
 }