[prep] arm64e rebased upstream-prep base (2026-04-23, u827651f)

[cypherair]

Summary

This draft PR tracks the rebased arm64e upstream-prep base branch on the CypherAir fork.

It is intentionally not the final upstream submission shape yet. Instead, it serves as the new internal staging branch after rebasing the arm64e experiment stack onto the latest rust-lang/rust main.

Current branch:

• codex/arm64e-upstream-prep-base-2026-04-23-u827651f

Base sync:

• fork main has been updated to upstream main at 827651f2200c
• this branch rebases the previous codex/arm64e-darwin-ptrauth-spike stack on top of that base

Why this PR exists

We do not want to keep preparing upstream-facing work on the old spike base, because that branch was 30 commits behind current upstream main.

This draft PR gives us a clean, rebased reference point on the fork before we split the work into a stacked upstream-prep series.

Scope in this draft

This branch still contains the full rebased experiment stack, including:

• Apple arm64e default ptrauth alignment work
• indirect-call authentication lowering
• function-pointer-as-data authentication work
• arm64e visionOS target/bootstrap carry
• bootstrap/build-helper fork-base detection fixes

Planned next step

From this rebased prep base, we will split the work into multiple stacked prep branches / draft PRs, roughly along these lines:

1. bootstrap/build_helper upstream-base detection fix
2. arm64e-apple-visionos target and docs
3. default Apple arm64e ptrauth semantics and user-facing restrictions
4. indirect call authentication lowering
5. function pointers used as data

Notes

• This PR is for fork-internal tracking and review preparation.
• It supersedes the old spike-based draft as the current working base for upstream prep, but it does not replace the old spike branch itself, which is intentionally preserved as a historical experiment snapshot.
• I have not yet rerun the full local validation suite after the rebase; the next branch-splitting step should do that per split branch.