lang: reject writes through constants (PLAN-CONST-AGG step 2, fixes 0116)

Any assignment / compound-assignment whose target chain is ROOTED at a
constant — a const-flagged global (array consts, #run consts) or a
module value const (struct consts incl.) — diagnoses 'cannot assign
through constant X' at compile time. A struct const's field write used
to compile and bus-error at runtime (issue 0116); scalars misfired
silently. A deref along the chain (p.*) breaks the root — pointer
writes stay the documented escape until the const-ness steps; a local
shadowing the const name stays writable.

Also: typed struct constants ('W : Color : Color.{...}') register —
the shape list skipped struct_literal, leaving the typed form
unresolved while the untyped one worked.

Examples: 1162 (all rejection shapes incl. the 0116 crash repro),
0178 (typed struct const reads + copy independence).

examples/expected/0178-types-typed-struct-const.exit

@@ -0,0 +1 @@
+0

examples/expected/0178-types-typed-struct-const.stderr

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+

examples/expected/0178-types-typed-struct-const.stdout

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+1 2 3
+copy=99 const=2

examples/expected/1162-diagnostics-const-write-rejected.exit

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+1

examples/expected/1162-diagnostics-const-write-rejected.stderr

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+error: cannot assign through constant 'WHITE' — constants are immutable (use a '=' global or a local copy for mutable data)
+  --> examples/1162-diagnostics-const-write-rejected.sx:15:5
+   |
+15 |     WHITE.r = 0;
+   |     ^^^^^^^
+
+error: cannot assign through constant 'K' — constants are immutable (use a '=' global or a local copy for mutable data)
+  --> examples/1162-diagnostics-const-write-rejected.sx:16:5
+   |
+16 |     K[0] = 5;
+   |     ^^^^
+
+error: cannot assign through constant 'K' — constants are immutable (use a '=' global or a local copy for mutable data)
+  --> examples/1162-diagnostics-const-write-rejected.sx:17:5
+   |
+17 |     K[1] += 2;
+   |     ^^^^
+
+error: cannot assign through constant 'N' — constants are immutable (use a '=' global or a local copy for mutable data)
+  --> examples/1162-diagnostics-const-write-rejected.sx:18:5
+   |
+18 |     N = 9;
+   |     ^^^^^^

examples/expected/1162-diagnostics-const-write-rejected.stdout

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+