feat(installer): implement Phase 3 — the patch tier

Two features need edits to stock ServUO sources, because the events they
depend on do not exist. This adds the rung ladder of PLAN.md §2.2.1, the
unsupported-version path of §2.2.2, and the record and cache Phase 4 will read.

Three decisions were not settled by the plan:

* The engine is fully native, with no `git`. §2.2.1 wrote rung 1 as "apply
  verbatim with git apply", but §1 chose the release tarball specifically so
  there would be no git on the shard host, and rung 2 needs a native applier
  regardless. Rung 1 keeps its distinct, stronger verdict — the whole file
  reproduced the diff's `index` pre-image, computed as a git blob SHA1 in
  process — while the write goes through the same code path as rung 2. On the
  real trees here that is not academic: the shipped .patch files are CRLF in a
  Windows checkout and two of their three targets are LF, so `git apply`
  refuses patches this applies correctly.

* Per-patch metadata is declared by the release, with a built-in fallback.
  Which patches form one all-or-nothing unit, which companion .cs follows
  which, whether a CORE rebuild is needed and what declining costs are not
  derivable from a diff. servuo-plugins now declares them; overlay v0.1.1 is in
  the current bundle and declares nothing, so a built-in copy stands in for it.
  A checked-in fixture of the release workflow's own jq output asserts the two
  descriptions are identical, so the repos cannot drift quietly.

* Pre-images are cached in the state directory. The tier edits files the
  operator owns, and `/etc/runicgateway/patches/originals/` is what turns "here
  are the hunks we added" into a revert anyone can verify — kept out of the
  ServUO tree, which uninstall has promised never to clean up.

Everything else follows §2.2.1: exact matching with only line-ending and
trailing-whitespace normalization, exactly one occurrence or it fails,
all-or-nothing per patch file and again per feature, and a byte-preserving
splice so nothing outside a hunk can be reformatted.

Verified against the ServUO 57.4 tree on this machine across four scratch
roots: a hand-patched tree (rung 0), a reverse-applied stock one (rung 1 on the
real EventSink.cs, its blob matching the patch's declared pre-image), a
mixed-rung feature, a tree with edits inside two patched regions (rung 3 —
nothing written, nothing held back applied, no companions copied), and a
non-57.4 tree both with and without the extra consent flag. Three consecutive
runs left install.json byte-identical and the cached pre-image still pre-patch.

Three reporting defects the live runs caught are fixed with tests: a dry run
and a held-back patch both claimed to be "applied", the core-rebuild warning
fired when nothing had been written and named a Scripts file as core, and a
declined tier announced the loss of features install.json showed as applied.
Refused patches are now cached too, since the refusal message names that path.

Refs: docs/installer/PLAN.md §2.2, §5 Phase 3

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

src/lib.rs

@@ -4,9 +4,9 @@
 //! record is `docs/installer/PLAN.md`; the operator-facing contract, written before this binary
 //! existed, is `docs/installer/INSTALL.md`.
 //!
-//! **This build implements Phases 1 and 2:** bundle resolution, ServUO detection and validation,
-//! the overlay sync, `install.json`, the uo-link sidecar and its service, and the token handoff.
-//! The patch tier (Phase 3) and `doctor`/`update`/`uninstall` (Phase 4) are not implemented, and
+//! **This build implements Phases 1 to 3:** bundle resolution, ServUO detection and validation,
+//! the overlay sync, the optional patch tier, `install.json`, the uo-link sidecar and its service,
+//! and the token handoff. `doctor`, `update` and `uninstall` (Phase 4) are not implemented, and
 //! each of them says so when reached rather than failing as though it were a typo.
 //!
 //! Exit codes: `0` success, `1` the run failed, `2` the arguments were unusable — the same
@@ -28,14 +28,17 @@
 
 pub mod bundle;
 pub mod cli;
+pub mod diff;
 pub mod install;
 pub mod net;
 pub mod overlay;
+pub mod patch;
 pub mod paths;
 pub mod record;
 pub mod service;
 pub mod servuo;
 pub mod sidecar;
+pub mod tier;
 pub mod ui;
 pub mod util;