fix(typecheck): instantiate generic fn type params (#135 slice 7)

[hyperpolymath]

What

#135 slice 7 — the root cause was not empty-array literals (those compile). It was a fundamental let-polymorphism bug.

check_fn_decl never registered a function's explicit <T> type parameters, so an uppercase param x: T lowered (via lower_type_expr TyCon fallthrough) to a rigid TCon "T". generalize ignores TCon, so every generic top-level function got sc_tyvars = [] — effectively monomorphic. The first call pinned T; any further instantiation failed Unify.TypeMismatch (T, Int), and use prelude::{…} import-checks failed transitively across result/option/collections.

How

Mirrors the existing let-generalization discipline: in check_fn_decl, before lowering, enter_level and register each fd_type_params name in ctx.type_env as a fresh tyvar at the deeper level; exit_level before the existing generalize so those vars become the scheme's quantified sc_tyvars. Scoped save/restore of type_env. Both extern and normal branches.

Plus genuine stdlib bugs surfaced once typecheck passed: prelude.affine map/filter/range/repeat/fold reassigned immutable let bindings → let mut.

Result

prelude.affine now compiles end-to-end (resolve→typecheck→codegen).

Verification

• id<T> at Int+Bool, generic fold from monomorphic sum, monomorphic fns — all compile.
• The fundamental typecheck change regressed none of the suite.
• 2 regression tests; full suite green (230 tests).

High-leverage: unblocks the typecheck wall; result/option/collections follow once slice 8 (module visibility) lands. Advances #135 (does not close). Refs #128, #135.

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 44 issues detected

Severity	Count
🔴 Critical	12
🟠 High	21
🟡 Medium	11

⚠️ Action Required: Critical security issues found!

View findings

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