…server v2

The v5 multi-lineage design lands as an OPT-IN second implementation
instead of replacing v1: --use-session-server grows an optional value
(bare flag or "v1" = the untouched linear server, "v2" = tree serving),
and the whole tree stack lives in miles/rollout/session/v2/. The v1
modules stay byte-identical to their pre-branch state.

v2 package (carried from the v5 line, imports adjusted only):
- session_state: always-branch serving over the forest — deepest attach
  point, suffix becomes the branch delta, non-extensions grow siblings or
  new roots; retry semantics moves entirely to the samples-op picker.
  Branch suffixes may carry client assistants (compaction); snapshot
  splice with zero-inheritance-root fallback on canonical-prefix
  divergence. Truncated-path extension is 409 (TruncatedGenerationError,
  defined here — v1 never raises it). --session-strict-append-only is the
  single-chain guard: branch shapes fail loud with attach diagnostics.
- assembly: per-leaf fold (compute -> truncate -> fold), default pick
  (temporal-supersession retry trim; roots never trimmed; non-retry-shaped
  trees 422) and default merge (exactly-once completion masking over the
  surviving set, rewards keyed by response id, fold < agent < server
  metadata layering); both replaceable via --session-sample-picker-path /
  --session-merge-function-path (sync-only, loaded in-process).
- core: v2 twin of SessionCore reusing v1's HTTP plumbing unchanged.

Shared seams, each inert for v1 by construction:
- sessions.py dispatches registry/core on the flag and forwards the
  collect body's "metadata" (agent semantic layer) only under v2.
- codec: encode/decode parameterized by a fields tuple; v1 default keeps
  the wire byte-identical, v2 adds reward + per-sample metadata
  (COMPUTED_FIELDS_V2) with conditional overlay semantics on decode.
- arguments: flag upgrade (nargs="?"), unknown values rejected, and the
  three v2 flags require --use-session-server v2.

Tests: v2 HTTP matrix (tree pins incl. deep/root divergence branching,
strict guard, truncation + compaction), session_state unit matrix, and
the samples-op suite (golden, multi-leaf trim/masking/rewards, hook
lanes) — all against a v2-flagged server; the restored v1 suites and the
byte-exact A-list (git diff against the pre-branch base is empty) pin
that the default path did not move.

Rebased onto the tito/session PR stack (#1628/#1777/#1778 +
#1759/#1760), with the stack's landed shape taken as authoritative:

- supports_midpath_assistant_rerender (True on the base class) moves in
  here with its consumer, the v2 suffix gate. No family sets it False
  anymore: the vendored DeepSeek-V3.2 encoder renders
  position-independently under drop_thinking=False, so the gate text no
  longer names it; a stub family pins the fail-loud branch.
- v2 serving fills SessionRecord.request_timestamp like v1, so folded
  samples keep their lifecycle req_ts across the wire.
- codec: COMPUTED_FIELDS_V2 is re-expressed over the ValueSpec table
  (SAMPLES_VALUE_SPEC_V2 superset), same wire semantics.
- v2 unit tests pin allowed_append_roles=["tool"] explicitly; the ctor
  default is tool+user since the append-roles PR.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>