the assembly root's bootstrap.py: create_at_pin still hides a branch's commits behind a new loose ref when HEAD is detached or a third spelling is packed, and _folded_twin does not fold NFC and NFD #962

Goal: bootstrap never makes, with checkout -b, a ref that a disk folding case or normalization reads as a branch that already exists.

Filed by lane openRepoShape-5, from rv896's independent review of #948 (#896) at 80353dd1 (#948 issuecomment-6252823490): its SHOULD 2, with w896's NFC/NFD checklist candidate. #948 lands first; this is what it leaves.

State at #948's head 80353dd1. _folded_twin asks only about HEAD's own ref, so create_at_pin checks no other spelling of the branch.

What happens. Measured on git 2.43 with packed refs, the fold emulated after the checkout (rv896's probe-packed.sh and probe-packed-c.sh):

• (a) HEAD on a packed Main holding commit M, and no main. bootstrap.py: a leg whose HEAD is on a branch that differs from the tracking branch only in case (Main for main) is left where it is, so on a disk that folds case branch -f main no longer moves the ref HEAD stands on (#896) #948 refuses this, as a twin.
• (b) HEAD detached at the pin, the same packed Main, and no main. create_at_pin runs checkout -q --no-overwrite-ignore -b main <pin>, which exits 0 and writes a loose main. On a disk that folds case, refs/heads/Main then reads as the pin. for-each-ref --contains M lists nothing, and only HEAD's reflog keeps M.
• (c) An unborn Main beside a packed MAIN. M is hidden the same way.
• NFC and NFD. An NFD name is not a twin of the NFC branch café, because str.casefold() does not normalize. On APFS, which reads both forms as one file, T7 then does bootstrap.py: on a disk that folds case, a leg on Main whose tracking branch main is behind the pin is moved under its own HEAD, and bootstrap says it is on main now over an index that undoes the pin #896's old harm, measured on the symlink emulation: fast-forwarded, then the leg was on café, which keeps its commits, then D NEW-CONTRACT.md;M README.md.

Direction.

• Before checkout -b, ask git for-each-ref refs/heads/ for any name that folds to <branch>, packed or loose. Refuse by name, with bootstrap.py: a leg whose HEAD is on a branch that differs from the tracking branch only in case (Main for main) is left where it is, so on a disk that folds case branch -f main no longer moves the ref HEAD stands on (#896) #948's keep line.
• Fold with unicodedata.normalize("NFC", ...) before casefold() in _folded_twin.
• Keep bootstrap.py: a leg whose HEAD is on a branch that differs from the tracking branch only in case (Main for main) is left where it is, so on a disk that folds case branch -f main no longer moves the ref HEAD stands on (#896) #948's rule that a twin is only ever left in place, never moved.

Verify.

• rv896's probes (b) and (c), and the NFD emulation, each run on bootstrap.py: a leg whose HEAD is on a branch that differs from the tracking branch only in case (Main for main) is left where it is, so on a disk that folds case branch -f main no longer moves the ref HEAD stands on (#896) #948's tests' set-up: nothing moves, and the words are exact.
• On a disk that keeps case, a real distinct Main gets the same true words.

Overlaps. #948 must land first. #944 (#932) and #958 (#430 slice D) edit the same file. bootstrap.py is a pinned copy, so every project re-pins it.

Size: S.

Milestone: none set; lane 3's call.

Lane: openreposhape-5 (openRepoShape-5)