new(tcl-lang.org/v8 + /tk/v8): Tcl/Tk 8.6 legacy (unblocks Tk + git-gui)

[tannevaled]

Tcl 9.0 broke API compat with 8.6; Tk 9.0 has a broken minizip Makefile target. Many downstream tools (git-gui, python-tk, BWidget) pin to 8.6.

This recipe adds Tcl 8.6 + Tk 8.6 as parallel pantry packages.

Unblocks #13057 (Tk) and #13060 (git-gui).

🤖 Generated with Claude Code

[tannevaled]

Diagnosis on the Tk 8 failures: it's the chicken-and-egg same-PR sibling issue. Tcl 8 builds fine (4/4 green), but Tk 8 fails on all four platforms with:

```
DownloadError: http: 404: https://dist.pkgx.dev/tcl-lang.org/v8/darwin/aarch64/versions.txt
```

(and matching 404 on the three other platforms). Tk 8's build resolver tries to fetch the Tcl 8 bottle from `dist.pkgx.dev`, but Tcl 8 is in this same PR — there's no published bottle yet.

I hit the same shape on font-util earlier in #13103 / #13106 and split them out: land the prerequisite alone first, then add the consumer in a follow-up PR once the bottle is on `dist.pkgx.dev`.

Proposal: let this PR land as Tcl 8 only (already green); the Tk 8 recipe can be a one-line follow-up PR after merge, with no new build problems to solve. I'm happy to author that follow-up.

Alternatively, if the brewkit test harness has a way to feed sibling-bottle output to a same-PR consumer (some kind of `linkPantry` or local-bottle override), I'd love to know — would unblock #13103/font-util too.