Release layout: snapshot-addressed releases with named symlinks

[marklubin]

Problem

Releases are currently stored under .synix/releases/<name>/ where <name> comes from --to local. Releasing twice to the same name overwrites the previous release — history is only preserved in a history/ subdirectory of receipt JSONs, but the actual materialized artifacts (search.db, context.md) are lost.

Additionally, pipeline.build_dir is a misnomer — nothing is written to it during synix build. It's only used as an anchor to locate .synix/ via synix_dir_for_build_dir(). The FlatFile adapter's default output_path="./build/context.md" further confuses this by writing outside .synix/.

Proposed Layout

.synix/releases/
├── <snapshot-oid>/        # immutable, snapshot-addressed
│   ├── search.db
│   ├── context.md
│   └── receipt.json
├── HEAD -> <latest-oid>/  # symlink to most recent release
└── local -> <oid>/        # named alias (from --to local)

Every release is immutable by snapshot ID. Named targets (local, prod) become symlinks. Re-releasing to the same name just repoints the symlink. Old releases are preserved and addressable.

What Changes

• release_engine.py — write to releases/<snapshot_oid>/, create/update symlink for named target
• search/adapter.py, flat_file_adapter.py — no change needed (they write to target_path which is passed in)
• synix search --release <name> — resolve symlink, same behavior
• synix releases list — show both snapshot-addressed releases and named aliases
• synix revert <ref> --to <name> — repoint symlink to existing snapshot release (instant if already materialized)
• SDK Release class — resolve symlinks transparently
• pipeline.build_dir — consider renaming to project_dir or removing entirely (separate from this, but related naming confusion)

Context

Surfaced during review of #96 (Chunk transform PR). Both the OpenAI and Claude automated reviewers flagged the build_dir naming confusion. The snapshot-addressed layout would also make synix revert instant when the target snapshot was previously released.