fix(math): correct comment-extremity sign in pca_project_cmnts (D1b)

[jucor]

pca_project_cmnts computed coefs = -scale * (1.0 + center), a literal,
untranslated copy of Clojure's synthetic vote value -1 (pca.clj:167-178).
Clojure stays in raw-Postgres convention (AGREE=-1) throughout, so -1 is
correct there. Delphi flips votes to its own convention at the Postgres ingress
(postgres_vote_to_delphi), fitting PCA on AGREE=+1 data, so the untranslated
-1 INVERTS comment extremity: |correct| = scale*|1-center| vs
|actual| = scale*|1+center| (equal only at center==0). A near-unanimous-AGREE
comment (center -> +1) read as maximally extreme (2*scale) instead of ~0; a
near-unanimous-DISAGREE comment (center -> -1) read as ~0 instead of maximal.

Fix: coefs = scale * (AGREE - center) (AGREE=+1, from utils.general) — the
faithful Delphi-convention port of Clojure's synthetic-AGREE projection.
Docstring rewritten to document the convention translation.

Tests (TDD, RED->GREEN):

• Replace the tautological test_pca_project_cmnts_formula (it re-derived the
  implementation's own buggy formula) with one deriving expected values
  independently from the AGREE constant.
• Add a behavioral sign test: unanimous-agree -> extremity 0,
  unanimous-disagree -> maximal.
• Add an integration test spying on the extremity E reaching priority_metric
  through _compute_comment_priorities (works despite the Clojure math.repness: priority-metric meta detection treats 0 as truthy, assigning META_PRIORITY^2 to every comment #2571 mirror — it
  inspects the argument, not the return), pinned to hand-derived 0 and 2*sqrt(2).
• Provenance comment in regression/utils.py: the regression CSVs are pre-flipped
  to Delphi convention by server/src/report.ts:388, so the regression path must
  NOT re-flip.

Output-inert today: priority_metric still returns META_PRIORITY**2 (the #2571
Clojure bug-mirror), so extremity affects no DynamoDB output yet and no golden
snapshots move. This is the extremity/PCA-parity groundwork the D12
priority_metric un-mirror is blocked on. Distinct from D1 (align_pca_signs,
temporal ±eigenvector stability).

Full delphi suite: 406 passed / 17 skipped / 47 xfailed / 0 failed.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

commit-id:b17cef9c

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Stack:

• fix(math): correct comment-extremity sign in pca_project_cmnts (D1b) #2614 ⬅
• docs(delphi): add math-algorithm history & routing-regression reference #2613
• fix(math): restore priority-based comment routing (regression from #1961) #2611

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⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[jucor]

Superseded by #2615 — spr re-created the PR after the commit was amended (provenance line-number fix). Identical change; #2615 is the live one in the stack.