test(api): add parallel-session stress test (I2)

[DemchaAV]

Summary

Wires up Track I2 from the v1.6.5→1.7 readiness taskboard. Drives 32 independent DocumentSession instances on a fixed-size thread pool through 4 iterations to catch race conditions in process-wide shared state (font registry, glyph cache, built-in node definitions, shape outline cache).

Two assertions

Test	What it pins
identicalSessionsInParallelProduceIdenticalLayoutGraphs	Every parallel render produces the same layoutGraph().toString() as a sequential baseline. Catches any shared mutable state racing in the prepare / measure pipeline.
independentSessionsInParallelProduceValidPdfBytes	Every PDF carries the %PDF magic, is at least 256 bytes, and size variance across threads is < 256 bytes — loose enough not to lock against legitimate timestamp drift, tight enough to catch corruption.

Why this scope

Each DocumentSession is single-threaded by contract — the stress test exercises a fleet of independent sessions, each owned by exactly one thread, not concurrent access to a single session. The guarantee under test is that the process-wide machinery (default font registry, glyph cache, built-in node definitions, shape outline cache) handles concurrent lookup safely.

Future-direction note: when Track I3 (DocumentSession.registry() immutable view, planned for 1.6.7) lands, this stress test will validate that the new contract continues to hold under concurrency.

Verification

$ ./mvnw -B -ntp test -pl . -Dtest=DocumentSessionParallelStressTest
Tests run: 2, Failures: 0, Errors: 0, Skipped: 0
Time elapsed: 1.572 s

128 layout renders + 128 PDF renders = 256 concurrent operations complete in ~1.6 s. Does not bloat CI.

Full suite re-run also green:

$ ./mvnw -B -ntp test -pl .
BUILD SUCCESS (~50s)

CHANGELOG entry added to v1.6.6 — Planned under ### Build.

Test plan

• New stress test green locally (2/2)
• Full canonical suite still green
• CI green on PR (including all matrix JDKs — race-condition detection works best on the higher-concurrency JDK 17/21/25 builds)
• Reviewer skim of the harness (one Executors.newFixedThreadPool(32) + invokeAll + assertions)
• (Out of scope) follow-up: extend the test to exercise shared-registry mutation paths once Track I3 lands the immutable registry() view