Add version API, pkg-config, SPDX headers, SECURITY.md, CHANGELOG.md

Project-hygiene pass toward being a properly citable, embeddable,
professionally-packaged C library rather than just working code:

- PACKFS_VERSION_MAJOR/MINOR/PATCH/STRING in include/packfs.h, the single
  source of truth for the project's version, plus a runtime pfs_version()
  (src/vfs.c, next to vfs_new/vfs_free) so a dynamically-linked consumer
  can check ABI/API compatibility without recompiling. Covered by a new
  assertion in tests/test_mem.c that the macro and the runtime function
  never disagree.
- packfs.pc.in + a `make install` rule that generates packfs.pc with its
  Version: field derived from PACKFS_VERSION_STRING via a Makefile-level
  grep/sed, never hand-maintained separately -- verified end-to-end with a
  scratch `make install PREFIX=...` + `pkg-config --cflags --libs packfs`
  + `make uninstall`, not just by reading the rule.
- SPDX-License-Identifier: MIT added to every src/*.c and src/internal.h
  (include/packfs.h already had one); the whole distributed source tree
  now carries consistent machine-readable license metadata.
- SECURITY.md, stating precisely what this project's containment and pack-
  integrity code actually claims as a security boundary (concept.md
  Section 6/7) versus what it explicitly does not (unenforced `mode`, no
  cross-process concurrency) -- not generic boilerplate -- with a real
  reporting contact rather than a placeholder.
- CHANGELOG.md (Keep a Changelog format), summarizing the real history in
  git log to date; explicitly notes no version is tagged yet.

Verified: clean `make all` + `make test` (all 6 binaries, including the
new version-check assertion), and a full ASan/UBSan sweep of all 6
binaries with zero real findings (one run hit the already-documented
DEADLYSIGNAL sandbox flake on test_pack_write_perf across all 3 retries;
re-verified directly afterward with 5/5 additional clean passes and timing
well under any plausible timeout, confirming it was the flake, not a
regression, before treating this as done).

Deliberately not done here, by the user's explicit choice: no
CODE_OF_CONDUCT.md, and no git remote/publishing -- this repository still
has neither, and both are decisions left to the maintainer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB

CLAUDE.md

@@ -4,7 +4,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 
 ## Repository status
 
-This repository contains a working v0 implementation of the `concept.md` specification: `include/packfs.h` (public API), `src/*.c` (implementation), `tests/test_*.c` (test suite), and open-source project scaffolding (`README.md`, `LICENSE`, `CONTRIBUTING.md`, `.github/workflows/ci.yml`), alongside the frozen `concept.md` and this file.
+This repository contains a working v0 implementation of the `concept.md` specification: `include/packfs.h` (public API), `src/*.c` (implementation), `tests/test_*.c` (test suite), and open-source project scaffolding (`README.md`, `LICENSE`, `CONTRIBUTING.md`, `CHANGELOG.md`, `SECURITY.md`, `packfs.pc.in`, `.github/workflows/ci.yml`), alongside the frozen `concept.md` and this file. Every file under `src/` and `include/` carries an `SPDX-License-Identifier: MIT` tag; `include/packfs.h`'s `PACKFS_VERSION_*` macros are the single source of truth for the project's version — `pfs_version()` (runtime) and `packfs.pc` (generated by `make install`) are both derived from them, never maintained separately.
 
 ## Build, test, and lint commands
 
@@ -27,7 +27,7 @@ Sanitizer builds are not wired into `make test` (they need per-file compilation
 
 - `include/packfs.h` — the entire public API.
 - `src/internal.h` — every internal type shared across `.c` files; read this first when touching implementation code.
-- `src/vfs.c` — the `Vfs` mount table itself (`MountSnapshot`, refcounted, atomically swapped) and the public API's dispatch-by-longest-prefix-match.
+- `src/vfs.c` — the `Vfs` mount table itself (`MountSnapshot`, refcounted, atomically swapped), the public API's dispatch-by-longest-prefix-match, and `pfs_version()` (trivially returns `PACKFS_VERSION_STRING`, kept next to `vfs_new`/`vfs_free` as the other whole-library-lifecycle entry points).
 - `src/upper.c` — the writable layer shared by `mem`, `dir`, and the overlay's upper side: `UpperSnapshot`/`UpperEntry`/`MutCell`, the persistent-treap index (`TreapNode` and `treap_*`/`node_*` — see "Known performance characteristics" below for why it's a treap and not a flat array), the structural-write functions (`upper_create`/`upper_mkdir`/`upper_remove`/`upper_rename`/`upper_copy_up`), the content-write fast path (`upper_cell_read`/`upper_cell_write`), the range-query API (`upper_visit_range`, used by `readdir` in both this file and `overlay.c`), and the standalone `mem`/`dir` `Backend` (`backend_mem_new`/`backend_dir_new`).
 - `src/overlay.c` — composes a `Pack` (lower, read-only) with an `UpperStore` (upper): copy-up, whiteouts, the merged view (`overlay_stat`/`overlay_readdir`), the append journal, and `vfs_sync`'s compaction.
 - `src/pack.c` — the on-disk pack format: load-time integrity validation, binary-search lookup/range-query, the compaction writer (atomic rename, exact-duplicate elimination via a `DedupSlot` open-addressing hash table over `(hash, size)` plus a `memcmp` verification before ever reusing a `data_off` — see "Known performance characteristics" below for why it isn't a linear scan), and the standalone read-only `pack` `Backend` (`backend_pack_new`) for mounting a pack with no writable upper layer at all.