Test cleanup: split oversized test files, fix three weak tests (tests only)

[floatdrop]

Summary

This is a second test-cleanup pass, after #101. It changes tests only: no production file changes, and no behaviour changes. It splits the test files that grew past 800 lines, many during the #102–#106 compliance work, and fixes three weak tests.

Commit	What
registry	track_test.go (1047 lines) → track_test.go (513, registry and entry basics) + ledger_test.go (450, the dedup ledger: §2.4.2/§9.1/§12.8/§12.9). Small shared claim helpers replace the repeated closures.
session	datastream_object_test.go (1020) → datastream_subgroup_test.go, datastream_subgroup_delta_test.go and datastream_fetch_test.go. sendSubgroup and writeObjects move to helpers_test.go.
relay	cross_relay_test.go (1378), session_pubsub_test.go (1076) and handler_fanout_test.go (907) → 8 topic files, each under 400 lines. Shared helpers move into harness_test.go and helpers_test.go, and near-identical tests share one body.
follow-ups	helpers_test.go (759) split by role into helpers_data_test.go and helpers_namespace_test.go, and the test fixes below.

Test fixes

• TestTrackStatus_ReplyForKnownTrack: it published the raw bytes "rtp-h265" as Track Properties. That isn't a valid Key-Value-Pair encoding, and the test only worked because those bytes happen to parse. It now publishes well-formed properties and checks that TRACK_STATUS_OK returns them byte for byte.
• TestPublish_ForwardedAliasDoesNotCollide: it registered only the first forwarded PUBLISH, so it relied on the colliding rtp PUBLISH arriving first. cam1's PUBLISH is forwarded too (§10.20). The test now registers each forwarded PUBLISH until the rtp one. I checked that it fails when every forwarded alias is forced to collide.
• TestIncomingFetchStream_ReadDecoded: an End of Range marker is compared by its Location only. Subgroup ID and Priority are not present on a marker (§11.4.4.2), so the test no longer pins the decoder's zero values.
• Citation: §11 becomes §11.2.1.1 in the invalid-status test.

Test set

• Same test names: the top-level test names are the same apart from these renames, from table-driving:
  • TestSubgroupObjectWithProperties → TestSubgroupObjectRoundTrip/with_properties
  • TestIncomingFetchStream_ReadDecoded_{Ascending,Descending,EndOfRange} → subtests of TestIncomingFetchStream_ReadDecoded
• Counts:
  • relay: 332 entries and 457 passes, before and after.
  • registry: 58 and 166, before and after.
  • session subtests: 167 before, 172 after.
• Stricter in places: some merged cases now assert more than before. For example, TestWriteObjectAtRejectsNonIncreasing now fails if a duplicate or lower ID is accepted, and some relay tests now also check payloads, the FIN or the Track Alias.

Checks

• Lint, suite, race: 0 lint issues; the full suite and -race pass. The per-package agents also stress-ran the moved tests under -race (15–30 runs).
• Reviews: moqt-reviewer checked each split commit and the whole branch: no production file changed, no assertion was lost, and the citations are correct.

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