refactor: give the frozen right header band its own width slack

[6pac-ai]

Warning

Stacked on #1274 (this PR's base branch is refactor/headers-width-slack) — it uses the HEADER_WIDTH_SLACK constant and the scroll-sync pin spec introduced there. Please merge #1274 first; GitHub will retarget this PR to master when that branch is deleted.

Resolves Q15 from the quirks triage (discussion #1247): under a left freeze, the right header band's width was cumulative — headersWidthR = max(sumR, viewportW) + headersWidthL — sizing the right band to include the entire left band's width for no stated reason.

Why it survived until now (the load-bearing part)

Every header band is built with left: -1000px on the columns container plus an injected .slick-header-column { left: 1000px; } rule, so each band's width must include 1000px of shift compensation or its columns overflow it. In the frozen path, the right band's compensation was delivered through headersWidthL (= sumL + slack) — which is why the triage classified Q15 as unknown-business-path and why a naive "drop the + headersWidthL" would genuinely break the right band (clipped header columns, scroll clamping short at full right scroll — the pin spec fails exactly there on the naive version).

The borrowed sumL on top of that compensation was the phantom part.

The change

One line: the right band now carries its own compensation —

this.headersWidthR = Math.max(this.headersWidthR, this.viewportW) + HEADER_WIDTH_SLACK;

The right header container gets sumL narrower (inert — it is clipped), and its post-offset extent becomes max(sumR + gutter, viewportW): exactly the regime the unfrozen band has always run in (its +1000 was likewise consumed by the -1000px offset all along). The frozen right band now matches that long-standing contract instead of exceeding it by an arbitrary, frozen-band-width-dependent margin.

This supersedes the triage's "defer normalization to the branch rework" position — the deferral existed because there was no safety net, and #1274's pin spec is that net.

Validation

• headers-width-scroll-sync.cy.ts (from refactor: single scrollbar add and named slack in getHeadersWidth #1274) passes unchanged — its frozen-grid checks (header scroll range covers body range, header reaches the body scrollLeft at full right scroll with no clamping, last-column header/cell alignment) exercise exactly this contract.
• example-frozen-columns-and-rows.cy.ts green (8/8).
• Full cypress suite: all specs passed — 652 tests, 650 passing, 2 pre-existing skips (3m46s).

(With this and #1274, every actionable width-arithmetic item from discussion #1247 is resolved; Q14's shift-compensation slack is now named and documented, and the Q15 entry in the tracking notes moves from keep-as-is to normalized.)

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