fix: compare optional payloads structurally, and finish the where-clause edges

[kacy]

Two live defects found while reviewing the sprint's work, plus the where-clause edges the review surfaced.

optional payload equality

The widened T? == T compare (#807) — and the pre-existing T? == T? compare — lowered every non-string payload to raw i64 equality. For Bytes and boxed enums the register holds a pointer, so two equal-content values silently compared unequal:

ob: Bytes? := bytes.from_string_utf8("abc")
ob == bytes.from_string_utf8("abc")   # was false, now true
s: Shade? := Shade.Tinted(3)
s == Shade.Tinted(3)                  # was false, now true

ir_emit_inner_value_eq (shared by both compares) now dispatches exactly the way the bare type's == does in ir_emit_value_binary: strings through __str_eq, bytes through bytes_eq, boxed enums through their per-enum structural helper, everything else i64.

The generated __enum_eq_<name> also gained a null guard after its identity fast path: a none optional's payload slot hands the helper a 0, and the widened compare reads the slot without branching on the flag — one null side now answers false instead of faulting on the tag read. (bytes_eq was already null-safe.)

where-clause edges

• A clause on an interface member signature parsed as expected kw_fn (E240) because parse_fn_sig never looked for it. The clause now parses through the same parse_optional_where_clause as functions and impl methods. Generic interface members themselves remain unsupported (E202 in the member's body, either bound spelling — pre-existing, now stated in docs/limitations.md); the invalid test pins that the clause reaches E202, not a parse stumble.
• E264 reported at the token after the whole bound list; it now lands on the offending parameter name.

noticed, not fixed

Generic interface members (E202) and impls on generic instances not satisfying bounds (E226, from #812's review) are both pre-existing, documented, and orthogonal.

what was tested

• New golden tests/cases/test_optional_payload_eq.pith: Bytes?/Bytes both orders, eq and neq, unequal content, Bytes? == Bytes?, boxed enum with payload and without, and the none-vs-variant case that faulted before the null guard (11 assertions).
• tests/cases/test_where_clauses.pith extended: a clause on a function with no return type, inline+clause bounds merged on the same parameter, and where used as an ordinary identifier; golden regenerated.
• New tests/invalid/where_interface_generic_member.pith (E202+E219); tests/invalid/where_unknown_param.pith confirms the E264 caret lands on the bad name.
• make check-invalid-only (54 passed), make run-regressions-only (371 passed), make bootstrap-verify fixed point after make refresh-bootstrap-seed.
• fmt/lint clean on all changed sources.