t27c: optional capture dropped, orelse unparsed, test-local reassign emitted as const, pointer param var rename #6358

Found by the t27b optionals lane (#6357), Refs #6063. Each is a t27c (parser or gen-zig) defect that makes the reference path refuse a well-formed spec:

1. if (x) |v| { ... } -- gen-zig drops the |v| capture; zig then reports v as undeclared. t27b refuses the shape as StmtIf(capture).
2. orelse does not parse in t27c.
3. In a test block, a plain assignment w = 9; to a name already declared with var is emitted as const w = 9;, a redeclaration zig rejects.
4. A mutated pointer parameter is renamed to var p = p_arg;, which zig rejects as "never mutated" when only p.* is written.

For 3 and 4, t27b currently passes such files while the reference does not compile them; they sit outside the 720 reference passes, so no count changes, but t27b should refuse them by name until the reference is fixed.

Boundary

• bootstrap/src/compiler.rs

User Scenarios

• Given a spec with if (x) |v| { use(v); }, when compiled by t27c, then the generated Zig must declare the capture v so the Zig compiler accepts it.
• Given a spec containing orelse in an optional chain, when parsed by t27c, then the parser must accept the keyword and produce a valid AST node.
• Given a test block where var w = 1; is followed by w = 9;, when codegen runs, then the reassignment must be emitted as a variable assignment, not a const redeclaration.
• Given a function taking a pointer parameter p: *i32 that mutates p.*, when codegen renames the parameter, then the generated Zig must not emit var p = p_arg; if only p.* is written.

Requirements

• FR-001: The gen-zig backend MUST emit a capture variable for optional if patterns so the Zig compiler sees the declared name.
• FR-002: The parser MUST recognize orelse as a valid keyword in optional expressions and produce a corresponding AST node.
• FR-003: In test blocks, assignments to already-declared var names MUST be emitted as mutable assignments, not const redeclarations.
• FR-004: When a pointer parameter is only mutated through dereference (p.*), the codegen MUST NOT emit a var p = p_arg; shadow that Zig flags as never mutated.

Success Criteria

• cargo test -p t27c -- optional_capture exits 0 and the generated Zig for if (x) |v| {} compiles without "undeclared identifier" errors.
• cargo test -p t27c -- orelse_parse exits 0 and a spec containing orelse parses without syntax errors.
• cargo test -p t27c -- test_reassign_const exits 0 and the emitted Zig for w = 9; inside a test block compiles without "redeclaration" errors.
• cargo test -p t27c -- ptr_param_rename exits 0 and the emitted Zig for a mutated pointer parameter compiles without "never mutated" errors.
• All 720 reference specs in specs/tools/tri/test.t27 still pass (t27c exit 0).