refactor(parser): typed clause provenance via ClauseIrBuilder + ClauseDisposition (U5-M1)

[matthewevans]

Replace the ad-hoc Vec<ClauseIr> accumulation in parse_effect_chain_ir
with an item-scoped ClauseIrBuilder that is the single authority for
ClauseIr construction — a _sealed field makes every external struct
literal a compile error, so the builder's one internal push is the sole
construction site. Each clause now carries a typed identity + provenance:

• ClauseId(u32): source-order identity minted by the builder.
• source: OracleUnitSource at the new honest SpanPrecision::ChainRelative
  tier — byte-exact within the effect chain and carrying the verbatim
  fragment, upgradeable to card-absolute once the document allocator is
  threaded through ParseContext (a later unit). The fragment-precision
  guard consults a widened carries_fragment (Exact | ChainRelative) rather
  than is_exact, so the new tier can carry its fragment without reopening
  the whole-document-fragment hole.
• ClauseDisposition (Emit / Continue / Special): folds the former
  absorbed_by_followup + special discriminants into one typed enum. The
  two continuations are orthogonal to the discriminant, so they are exposed
  as role-named accessors that span variants: followup() (prior-patch;
  carried by Emit + Continue) and intrinsic() (self-patch; carried by
  Emit + Special).

absorb_clause re-mints a nested chain's clauses through the same
.clause(...).push() path, preserving the single-construction invariant.
The dead is_otherwise field is dropped (zero readers, verified).

Ten identical Effect::Unimplemented { description: None } placeholders (the
.parsed field of Special-disposition clauses, where the real semantics
live in the disposition) are folded into one placeholder_parsed_clause
helper — byte-identical, but DRY and distinct from the Effect::unimplemented
parse-gap constructor (which forces description: Some).

Faithful-by-intent, no behavior change: the IR snapshot parity tests hold
with zero _lowered.snap movement (63 snapshot tests + the full parser
suite green under -D warnings). locate() verbatim-fragment fallback rate:
1/4859 (0.02%) over the 568-card fixture.

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Parse changes introduced by this PR

✓ No card-parse changes detected.