[FEAT]: Durable xdist shard transport primitives (no wiring)

[nina-msft]

Description

First PR in a stack of two that adds durable result transport for pytest-xdist, closing two gaps in the current in-memory workeroutput transport:

• Durability — a worker killed mid-run currently loses all of its finished results (nothing is written until a clean pytest_sessionfinish).
• Granular size cap — a single oversized transcript currently replaces a whole worker's payload with a truncation marker, discarding every other result from that worker.

This PR lands only the primitives for the fix. Every symbol is a pure addition to rampart/pytest_plugin/_xdist.py with no call sites — existing behavior is completely unchanged. It establishes the vocabulary the wiring PR consumes, so it can be reviewed small and green on its own.

What's added

Symbol	Purpose
ShardWriter	Append-only, flush-per-result JSONL writer. A worker's finished results hit disk immediately, so they survive an abrupt worker death (e.g. SIGKILL). An oversized record is written as a marker line naming its nodeid/index instead of discarding the whole shard.
read_worker_shard	Controller-side recovery. Parses a worker shard line by line, tolerates a truncated/partial final line, and surfaces corrupt/oversized records as DroppedRecord entries so the caller can mark the run incomplete. nodeid/index are taken authoritatively from the record envelope, mirroring deserialize_worker_data's existing trust posture (never trusting values embedded in the serialized result body).
shard_eligible	Gates shard transport to runs where every --tx gateway is a local popen worker (shared filesystem). Remote (ssh=/socket=) or proxied (via=) topologies are ineligible and will fall back to workeroutput in PR B.
SHARD_DIR_KEY	Stash key constant for the controller→worker shard-directory channel used in PR B.

The full-fidelity Result round-trip reuses the existing _serialize_result/_deserialize_result pair, so shards carry the same fidelity as the current transport.

🔜 Coming in PR B — wire durable transport (the behavior change)

See draft here: nina-msft#3

PR A is inert on its own. The follow-up (dev/nina-msft/xdist-durable-transport, stacked on this branch) turns the primitives on:

• Controller creates the shard directory and hands it to workers via a pytest_configure_node channel (SHARD_DIR_KEY).
• Workers append each result to their shard as it completes and stamp "transport": "shard".
• The atomic flip: workeroutput is slimmed to a completeness sentinel (result bodies removed) and handle_testnodedown switches to reading results from shards — these two changes must land together, which is exactly why they're held out of this PR.
• Mandatory remote/non-eligible fallback to workeroutput (so single-process and remote runs are unaffected).
• Slow pytester tests proving the wiring: worker-kill recovery, oversized-record isolation, crash-item-absent, and clean-run parity; plus the docs/usage/xdist.md update.

Splitting here keeps the "flip the source of truth" as one reviewable semantic change and keeps this PR a zero-risk, additive foundation.

Breaking changes

None. No existing code path calls any of the new symbols; behavior is identical to main.

Checklist

• pre-commit run --all-files passes
• Tests added or updated for changes
• Documentation updated