Documentation: Clarify codegen environment rules for top-level const and function bindings

[hyperpolymath]

Describe the bug
Documentation needs to clarify that both top-level consts and fns must be present in the codegen environment. Current docs may only discuss top-level fns.

To Reproduce
Look for existing documentation on module environment in codegen.

Expected behavior
Section covering environment population rules for codegen makes explicit how top-level const and function bindings are represented and threaded.

Additional context

• Linked to the implementation and test issues from parent Codegen.UnboundVariable for top-level const bindings (typecheck OK, compile fails) #73.
• See bin/main.ml and lib/codegen.ml for intended behavior.

[hyperpolymath]

Resolved by PR #91 (merged as c392e49).

The SPEC.md changes:

• Section 2.1 (Program Structure): top_level production now lists const_decl, extern_fn_decl, and extern_type_decl alongside fn_decl so the grammar matches what the parser actually accepts after PR feat: typed-wasm xmod + Node backend + extern + VS Code (#35, #42) #90 landed extern support.
• Section 2.9 (Constant Declarations): new EBNF and prose noting that const initializers must reduce to Wasm constant expressions, with a forward-reference to section 8 for the codegen environment encoding.
• Section 2.10 (Extern Declarations): new EBNF for extern fn / extern type, with the runtime contract (extern fn → (import "env" "<name>" ...), extern type → no Wasm artifact).
• Section 8 (Codegen Module Environment): new section documenting the func_indices : (string * int) list table. Positive k ≥ 0 is a Wasm function index (defined functions or extern-imported functions); negative k < 0 encodes -(global_idx + 1) for top-level constants. Includes per-case gen_decl behaviour for TopFn (defined), TopFn with FnExtern body, and TopConst. Explicitly notes the call-site ExprApp lookup currently emits call k unconditionally — decoding the negative sentinel back to global.get is the implementation half tracked under Codegen.UnboundVariable for top-level const bindings (typecheck OK, compile fails) #73.

The companion lib/codegen.ml doc-comment on the func_indices field was updated to mention both TopFn paths (defined and extern) and that insertion is in source declaration order.

Note: PR #94 introduces a more detailed docs/specs/codegen-environment.adoc that goes deeper on cross-module threading and non-WASM target behaviour. That landing will subsume the SPEC.md §8 content here; this issue is resolved at the level it asked for (clarify that both consts and fns must be present in the codegen environment, document the population rules).