Refresh nested manifests before benchmark builds

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Depends on #1612 for the Julia 1.10-compatible Testing manifest.

Summary

• Resolve a nested benchmark manifest before instantiation when the running Julia minor is the same as or newer than the manifest's Julia minor.
• Preserve manifests produced by a newer Julia when running on an older Julia, whose resolver may not know the newer stdlibs.
• Add a regression test using a stale minimal manifest.

Root cause

Julia 1.12 no longer bundles MbedTLS_jll, while the checked-in Julia 1.11 Testing manifest records it as a stdlib without a source tree. Pkg.instantiate() therefore leaves no package path for the subsequent Pkg.build() call.

A full bisect found cc4abfcee7f55d5be8be72060f382c1de5d1742f as the first bad commit because it introduced and activated the nested Testing environment. Resolving the manifest on same-or-newer Julia versions converts historical stdlibs such as MbedTLS_jll to normal registered packages before building.

Verification

• Julia 1.12: full Pkg.test() passes on this branch, including ExplicitImports, the stale-manifest regression, and the real woven benchmark test.
• Julia 1.10: full Pkg.test() passes when stacked on Regenerate Testing manifest with Julia 1.10 #1612.
• Runic 1.7.0: full-repository check passes.

[ChrisRackauckas-Claude]

Investigation scratchpad

Task prompt: independently reproduce the clean-master Julia 1.12 Pkg.test() failure, bisect it, fix the underlying nested-environment behavior without skipping tests, and verify the actual test locally.

• Clean base: 0498ef2519fdc2f60a1a2d78502f971bdf0b3448.
• Independent reproducer: Julia 1.12 with an isolated depot, activating benchmarks/Testing and running Pkg.instantiate(); Pkg.build().
• Reproduction result: exit 1, Failed to find path for package MbedTLS_jll.
• Bisect range: clean master bad, cc4abfce^ good; automated predicate ran the public Pkg.instantiate(); Pkg.build() path and classified only the exact MbedTLS_jll path error as bad.
• Bisect result: first bad cc4abfcee7f55d5be8be72060f382c1de5d1742f (Move test.jmd into its own project folder). That commit introduced the nested environment and made the test activate its manifest.
• First repair probe: resolving before instantiation on Julia 1.12 added a source tree for the historical MbedTLS_jll stdlib and allowed Pkg.build() to complete.
• Compatibility iteration: unconditional resolution is invalid when an older Julia reads a newer manifest; Julia 1.10 cannot resolve the manifest's Julia 1.11 StyledStrings stdlib pin. The implementation therefore resolves only on the same or a newer Julia minor.
• Added regression: a temporary project with a deliberately stale manifest must be resolved and instantiated by weave_file.
• Julia 1.12 full test result: Explicit Imports 2/2, stale-manifest regression 2/2, real weave_file 1/1; Pkg.test() passed.
• Julia 1.10 validation stacked on Regenerate Testing manifest with Julia 1.10 #1612: Explicit Imports 2/2, stale-manifest regression 2/2, real weave_file 1/1; Pkg.test() passed.
• Formatting: Runic 1.7.0 full-repository check passed.

No tests were skipped, disabled, loosened, or silenced.

[ChrisRackauckas-Claude]

CI note: the immediate Spell Check failure is the two pre-existing SerPSO findings on clean master. It is unrelated to this two-file diff and is fixed separately by #1614.

[ChrisRackauckas-Claude]

Post-timer CI review:

• Runic Format and Runic Suggestions pass.
• Julia Tests: Explicit Imports (2/2) and the new stale-manifest regression (2/2) pass. The only failure is the pre-existing Julia 1.10 read of the Julia 1.11 Testing manifest: Could not locate source code for StyledStrings. This is the exact prerequisite addressed by Regenerate Testing manifest with Julia 1.10 #1612; the full Julia 1.10 Pkg.test() passes locally when this branch is stacked on Regenerate Testing manifest with Julia 1.10 #1612.
• Spell Check: the exact two clean-master SerPSO findings remain; they are addressed by Use descriptive SerialPSO benchmark alias #1614.

No branch-local CI failure was found, so no additional change was pushed.