feat(lang): a module's private names are its own

A private function, var or const no longer collides with the same
spelling in another module, in no module or in the runtime: where two
meet, each private one is renamed for its module (seed$shop), with every
reference its module writes that no local shadows. Exported names stay
one namespace. Nothing is renamed without a clash. Reseed.

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

examples/modules/private_twins.ludic

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+# private_twins.ludic — L3: a module's private names are its own. shop and weather each have a
+# private `seed`, `dice` and `LIMIT`, and the program has a `seed` of its own; none of them collide,
+# and each module's code reaches its own.
+#
+# Running it prints:  105 41 7
+import "twins/shop"
+import "twins/weather"
+program PrivateTwins {
+  function seed() -> int {
+    return 7
+  }
+  entry {
+    print(`{shop_seed()} {weather_seed()} {seed()}`)
+  }
+}

examples/modules/twins/shop/index.ludic

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+# shop/index.ludic - private names a neighbour also uses: seed, dice, LIMIT
+module shop
+var dice: int = 100
+const LIMIT: int = 5
+function seed() -> int {
+  return dice + LIMIT
+}
+export function shop_seed() -> int {
+  return seed()
+}

examples/modules/twins/weather/index.ludic

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+# weather/index.ludic - the same private spellings, its own; a local named dice is its own too
+module weather
+var dice: int = 20
+const LIMIT: int = 2
+function seed() -> int {
+  return dice * LIMIT
+}
+function roll(dice: int) -> int {
+  return dice + 1
+}
+export function weather_seed() -> int {
+  return seed() + roll(0)
+}

examples/rejected/exported_twins.ludic

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+# L3: two modules that both EXPORT a name clash - only private names are a module's own
+import "exported_twins/a"
+import "exported_twins/b"
+program ExportedTwins {
+  entry {
+    print(seed())
+  }
+}

examples/rejected/exported_twins/a/index.ludic

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+# a/index.ludic - exports seed
+module a
+export function seed() -> int {
+  return 1
+}

examples/rejected/exported_twins/b/index.ludic

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+# b/index.ludic - exports seed too: exported names are one namespace
+module b
+export function seed() -> int {
+  return 2
+}