BlizzardSkin: reassert Buy's anchor and bound the tab-gap scale

[dfrisone]

Fix from Svart. His GitHub account is currently suspended, so this is being submitted on his behalf.

What does this PR do?

Fixes the Auction House's bottom tab row (Buy / Sell / My Auctions) drifting badly out of position after swapping between the base UI and another AH addon skin (e.g. TSM).

Blizzard's tab row is a chain, not three independent anchors: Sell anchors off Buy's edge, Auctions off Sell's. Two bugs compounded here:

1. The re-skin only reasserted anchors for the 2nd+ tabs. A displaced BuyTab -- left that way by the other addon after closing its own AH skin -- threw off everything chained after it, since nothing ever put Buy itself back.
2. The gap-width calculation divided by a tab's GetEffectiveScale() with no bounds check. A near-zero leftover scale (also left behind by the other addon hiding a tab) turned what should be a ~1px gap into hundreds of pixels.

Fix: explicitly reassert BuyTab's own default anchor before chaining the rest of the row off it, and bound the effective scale to 0.1-10 before dividing by it.

How was it tested?

Author-tested by Svart; not independently retested in-game by me before opening this on his behalf.

Checklist

• New settings default OFF (no behavior change without opt-in) -- N/A, no new settings
• Zero cost while disabled: no events registered, no polling, no hooks doing work, no frames built -- N/A, this only runs when the AH re-skin already runs
• Cheap while enabled: event-driven (no polling, no timer-based logic, no per-frame allocations) -- both changes are a handful of extra table reads and one anchor call, only when the tab row is (re)built
• No writes onto Blizzard-owned frames (weak-table pattern used); HookScript/hooksecurefunc only, never SetScript on Blizzard frames -- the anchor reassertion is ClearAllPoints/SetPoint on Blizzard's own AuctionHouseFrameBuyTab, the same pattern the surrounding re-skin already uses on every other tab in the row
• Tested in-game on live; no version gates or pre-Midnight APIs added -- see note above