Add per-user learned machine profile; fix methodology honesty issues

Replaces the flat additive batch-size correction with a per-user
multiplicative pace factor learned from each account's own logged
roasts (shared/learn.js, GET /api/machine-profile), plus a lot-scoped
"last refine" auto-suggestion. Also fixes the reference data being
framed as "your own roasts" on a now-multi-user product, and
reclassifies the drying/Maillard sanity checks as informational since
they're algebraically derived from the DTR check rather than
independent (yellow = 0.56 x first crack, not entered separately).

Bug fixes found by an adversarial Opus review of the first pass:
- Unicode minus sign (U+2212) broke duration parsing against the
  app's own generated refine-suggestion text
- Printed sanity-checks table still showed a bare pass/fail glyph for
  the now-informational drying/Maillard rows
- Printed time-ledger box didn't show the pace multiplication step,
  so it stopped reconciling by hand once pace != 1
- Field 6.4 (manual batch correction) was double-counted: excluded
  from the learned-pace fit but added back after the multiplication
- Learned pace had no outlier rejection or hard clamp
- computeLedger had no test coverage
- Batch-size help copy overstated what the pace factor models (it's
  a single blanket ratio, not conditioned on batch weight)

A follow-up Opus pass also caught the per-user profile cache
surviving logout/account-switch in a shared browser; fixed by
sweeping it alongside the existing plan-draft cleanup.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

public/worksheet.css

@@ -77,6 +77,7 @@ tbody tr:nth-child(even) td{ background:#faf7f4; }
 td.num,th.num{ text-align:center; font-variant-numeric:tabular-nums; white-space:nowrap; }
 .mono{ font-family:"SF Mono",Menlo,Consolas,monospace; font-size:7.6pt; }
 .tight td{ padding:.85mm 1.15mm; }
+.wsfoot{ font-size:6.8pt; color:var(--mid); line-height:1.35; margin:.8mm 0 0; }
 
 /* ─── Callouts ────────────────────────────────────────────────────────── */
 .why{