docs(spec): thin SPEC.md §8 to forward-reference codegen-environment.adoc

PR #94 contributes docs/specs/codegen-environment.adoc as the canonical
.adoc spec for the codegen module environment. This commit removes the
duplicate inline §8.1/§8.2 in SPEC.md and replaces the section body with
a 2–3 sentence pointer to the canonical file, keeping the §8 heading as
an anchor so existing cross-references still resolve.

§2.9 (const_decl EBNF) and §2.10 (extern_*_decl EBNF) are retained — they
are grammar productions and belong in SPEC.md.

Refs #89 — companion to PR #94 (canonical doc) and PR #92 (extern impl).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

docs/specs/SPEC.md

@@ -543,52 +543,14 @@ Compiles to (ownership removed):
 
 ## 8. Codegen Module Environment
 
-This section describes how the WebAssembly code generator (`lib/codegen.ml`)
-builds its name environment. It is implementation documentation aimed at
-contributors; the language semantics are fully specified in §2–4.
-
-### 8.1 Name Environment (`func_indices`)
-
-The codegen context maintains a single association list
-
-```ocaml
-func_indices : (string * int) list
-```
-
-that maps every top-level name visible at later declaration sites to an
-integer key. Two distinct kinds of binding share this table:
-
-| Source declaration | Key value | Meaning |
-|--------------------|-----------|---------|
-| `fn f(…) { … }` | `k ≥ 0` | WebAssembly function index (imports + defined functions, combined) |
-| `const C: T = e` | `-(g + 1)`, where `g` is the global's index in the Wasm `globals` vector | Negative sentinel reserved for constants |
-
-Sign-based partitioning is deliberate: `k ≥ 0` decodes directly as a Wasm
-`funcidx`, and `k < 0` recovers the global index as `g = -(k + 1)`. A
-single integer per name keeps the lookup uniform across both kinds of binding.
-
-**Population.** Top-level declarations are visited in source order by
-`gen_decl`, which is folded over `prog.prog_decls` from `generate_module`.
-The two relevant cases are:
-
-- `TopFn fd` — registers `(fd.fd_name.name, func_idx)` in `func_indices`
-  *before* generating the function body, so the body may recursively refer
-  to its own name.
-- `TopConst tc` — generates the global initializer, appends the global to
-  `ctx.globals`, then registers `(tc.tc_name.name, -(global_idx + 1))` in
-  `func_indices`.
-
-Because population is strictly single-pass and in declaration order,
-forward references (to either functions or constants declared later in the
-file) are not supported by the current backend.
-
-**Call-site lookup.** The `ExprApp (ExprVar id, _)` branch of `gen_expr`
-consults `func_indices` to translate a direct call into a Wasm `call k`
-instruction. Decoding the negative sentinel back to a `global.get` —
-needed to make a bare `const` identifier usable inside another top-level
-declaration's body — is tracked as a known gap in issue #73. The encoding
-documented in this section is the data layout the fix relies on; the
-call-site decode path will land alongside that fix.
+The WebAssembly code generator's name environment — the `func_indices`
+sign-sentinel encoding for `fn`/`const` bindings, the population order, the
+call-site decode path, and (once #92 lands) the treatment of `extern fn` /
+`extern type` declarations — is documented in full in
+`docs/specs/codegen-environment.adoc`.
+
+That spec is the canonical implementation reference; this section is a thin
+anchor preserved so cross-references to "SPEC.md §8" continue to resolve.
 
 ## Appendix: Grammar Reference