[A13xB0]

From upstream

This PR brings in upstream Kpa-clawbot/CoreScope#2149 by @n30nex, unchanged. It is open upstream at the time of writing and addresses upstream issue Kpa-clawbot#2148. Both commits are cherry-picked with -x, so each commit message names its upstream commit:

• 532436e6 test: respect native permissions and fixture release context (upstream 2403d28f)
• c663f6a7 perf(server): reduce retained observation allocation size (upstream 1b66d388)

They applied to our master (3e21b17) with no conflicts. Nothing in this PR was written for the fork; if upstream changes Kpa-clawbot#2149 before merging, this PR should be refreshed from it.

What it changes

StoreObs (one per retained observation) had tsParseOnce sync.Once between two fields, and the padding put the struct at 200 bytes, so each observation landed in Go's 208-byte allocation class. Moving tsParsed time.Time ahead of the sync.Once packs it to 192 bytes, the 192-byte class. The second commit makes three tests portable (Windows symlink privilege, POSIX file modes, release-notes fixture context).

Performance

Upstream's own BenchmarkStoreObsRetainedMemory (100k observations), 10 runs each on master and on this branch:

                          │   master    │            this PR             │
                          │   sec/op    │   sec/op     vs base           │
StoreObsRetainedMemory-12   15.71m ± 1%   15.69m ± 2%  ~ (p=0.579 n=10)

                          │ retained-B/obs │ retained-B/obs  vs base           │
StoreObsRetainedMemory-12        208.0 ± 0%       192.0 ± 0%  -7.69% (p=0.000 n=10)

Scale on our deployment: production holds about 167k observations in memory, so this saves about 2.7 MB of heap there. It is a small, free win; the larger memory items are tracked separately (subpath index, analytics recompute allocations).

Tests

• TestStoreObsMemoryFootprint (from upstream) fails if StoreObs grows past 192 bytes on 64-bit targets.
• make test: 16 of 16 modules pass.
• Frontend suites: 194 of 194 pass. On our master, tests/unit/test-issue-1956-release-routing.js failed locally because it expected CI's GITHUB_REF_NAME; the second commit fixes that, so this branch is the first where every local suite passes.

Review notes

• Pure field reorder; no behaviour change. sync.Once must not be copied after first use, and nothing copies StoreObs by value (it is always held by pointer).
• Test changes come from upstream as-is.

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