ieee: the preprint draft-stage mirror + post-#515 pins

[ronaldtse]

The pubid-ts mirror of the preprint draft-stage wave (pubid#515, testsuite#41, grammar#31/#32 — all merged).

Ruling: Preprint is a draft stage of the C2 code (the NESC's own term for its pre-edition drafts), not a variant marker.

• Model (src/flavors/ieee/model.ts): NescPreprintClass (kind: "nesc-preprint", codeColumns, renders IEEE Std C2.YYYY.Preprint); the URN generator gains a trailing preprint component (urn:ieee:ieee:C2:2012:preprint) so a preprint cannot share the published edition's URN.
• Grammar (src/flavors/ieee/grammar.ts): nesc_c2_preprint (dotted / dash / catalogue slash-tail spellings) and nesc_preprint_proposal (verbose Preprint Proposal(s) for the YYYY Edition of …, longest-literal-first) slot into nesc_identifier before the C2-standard forms; nesc_delegation now consumes the optional IEEE Std prefix BEFORE its lookahead guard (a guard anchored at position 0 rejected every prefixed catalogue spelling) — the same fix the PARG grammar took.
• Builder (src/flavors/ieee/index.ts): buildNesc routes on the preprint capture before the draft/variant/code checks (the C2-led preprint tree also carries code).
• Preprocessor (src/flavors/ieee/preprocessor.ts): the /Preprint strip is removed — it was data loss, collapsing distinct documents onto their published editions (the same line the Ruby parser dropped in #515).
• Pins: TESTSUITE_REF → testsuite#41 (358665808c4249cf0d22fec57dedb54f8f6b6c84, the preprint corpus round); PARG_GRAMMAR_REF → grammar#31 (d182eb9eaa2199663bfaec8b889cbf6339bdc2a0).

All 5 new nesc_preprint corpus cases pass through the hand grammar (parse, render, URN, aliases); the pend ledger is unchanged at 477 with 0 satisfied.

URN-parity fixture (test/fixtures/urn-parity.json.gz): regenerated against pubid#515 — the corpus grew by the #215-era adoption rows the stale fixture never covered. That exposed 26 pre-existing ieee parseUrn divergences (ISO/IEC/IEEE adoption faces the ts grammar rejects in the IEEE-face reconstruction, the reference's own garbage draft.D= renders, separator variants like 1076.CONC vs 1076-CONC); ieee joins nist in the counted KNOWN-DIVERGENT set (9,476/9,502 clean) until the parseUrn parity port lands. None of the 26 involve preprint.