refactor: dedupe and flatten the sprint's checker, emitter, and parser additions

[kacy]

A readability pass over the code #804-#812 added, provably behavior-neutral: the refactored compiler emits byte-identical IR to the pre-refactor compiler across 552 corpus files, including the compiler's own 5.4 MB closure. Net -16 source lines.

checker

• The E263 duplicate-method block tracked (key, kind) pairs through a comma-joined string probed with substring contains, plus a cross-registry has_method_declaration read that was provably equivalent to a same-map lookup. It is now one Map[String, Bool] keyed "<key>#<kind>" and a named dup_check_applies — and the previously implicit third case (an unresolvable impl target is exempt) is stated in a comment.
• optional_plain_comparison's two mirrored halves and the tail of argument_widens_to_optional were the same predicate written three times; extracted as optional_accepts_plain.
• check_generic_enum_variant_construction: removed an unreachable E212 branch (the sole caller's enum_decl_has_variant guard is the gate — now said in a comment) and its now-unused enum_name parameter; replaced an inline copy of argument_expression_index with the helper.
• New normalized_method_name retires two hand-rolled leading-dot strips.
• propagate_empty_argument_collection_type: a condition whose both arms reached the same return now calls collection_literal_coerces for its recording effect and says so.
• Hoisted a repeated generic_enum_construction_base call in the annotated-binding path.

emitter

• ir_emit_return_value shed the if true: wrapper the extraction in fix: give a match on an rc-payload optional exact ownership #809 left behind (pure dedent).
• The verbatim-duplicated release-then-retain dance for match bindings (optional subject vs enum payload) is one helper, ir_bind_owned_rc_payload, carrying the full untracked-name rationale that previously lived only on the enum copy.
• The track-or-poison prepass policy, written out three times in ir_collect_string_bind_names, is one helper, ir_prepass_track_or_poison; the optional-match hook also gained a comment stating it sees only top-level arm patterns (unlike the pat_variant hook, which recurses).
• The two 200+-character mirrored conditions gating the widened compare collapse to ir_plain_compare_operand_ok.
• ir_emit_optional_optional_compare / ir_emit_optional_plain_compare shared two verbatim blocks; now ir_emit_inner_value_eq and ir_emit_guarded_eq.
• The syntactic tuple-literal release gate names its condition (is_tuple_literal) and its comment now says explicitly that the second "tuple" is an AST node kind, not the rc kind spelled the same; the stale "therefore still leaks its box" sentence on ir_owned_arg_kind_releases is corrected to point at the caller that now handles it.
• ir_emit_match_arm_return drops an unused pub and its trailing flag-set is annotated as belt-and-braces.

parser

• parse_fn_decl built its child list, then rebuilt it around the where clause with an index walk and a ret_node sentinel ordering dance; it now builds the list once, after the clause. The bound-list grammar is one parse_bound_list shared by the inline and where spellings so they cannot drift, and the merge is a named merge_generic_param_bounds.
• The merge loop's inverted-reading for g_idx, gi follows the value-then-index convention (gp_idx, gp_i), stops at the first match, and generics is declared mut (it was written through an index while immutable — E216 only covers ident targets, which is itself worth a look someday).

what was tested

• Byte-identical IR A/B: dumped IR for all tests/cases, all examples, and self-host/ir_driver.pith with the pre- and post-refactor compilers — 552 files, zero differences; the compiler's own closure is byte-identical, so codegen provably did not change.
• make bootstrap-verify — fixed point holds after make refresh-bootstrap-seed (the seed diff is the recompiled refactored source itself, per the A/B above).
• make check-invalid-only (53 passed) — E262/E263/E264/E226 rejections all unchanged.
• make run-regressions-only (370 passed).
• fmt run on all five files (no drift); lint ok on all five.