Add shared quilt settings and patch queue documentation

src/quiltrc fixes refresh output (no Index: lines or timestamps, a/ b/
prefixes, diffstat) so that every contributor produces identical
patches. src/README.md describes driving the queues with it, the policy
of refreshing a component's whole queue when its version changes, and
how gcc-final stacks on gcc.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/README.md

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+# Patch queues
+
+Each subdirectory here is a [quilt][] patch queue for one component: a
+`series` file listing patches in application order, plus the patches. The
+Dockerfile applies each queue to a freshly unpacked source tree. Order
+comes from `series`, never from file names.
+
+## Working on a queue
+
+Drive a queue straight out of this directory by pointing `QUILT_PATCHES`
+at it from inside an unpacked source tree, with `--quiltrc` selecting the
+shared settings in `quiltrc` so that everyone produces identical patches:
+
+    $ W=/path/to/w64devkit
+    $ tar xf $W/source/cmake-4.4.3.tar.gz
+    $ cd cmake-4.4.3/
+    $ export QUILT_PATCHES=$W/src/cmake
+    $ quilt --quiltrc $W/src/quiltrc push -a
+
+From there use quilt as usual (`push`, `pop`, `refresh`, `new`, `edit`,
+etc.). `refresh` writes directly into this directory, so check `git
+status` afterward.
+
+Always start from a freshly unpacked tree. Quilt records applied patches
+in `.pc/`, and a tree reused with a different `QUILT_PATCHES` acts on the
+queue recorded there.
+
+## Updating a component
+
+When a component's version changes, refresh its entire queue against the
+new source in the same commit:
+
+    $ quilt --quiltrc $W/src/quiltrc push -a --refresh
+
+Read the output before committing. "Hunk succeeded at N (offset M)" is
+harmless and is what the refresh cleans up. "with fuzz" means patch
+discarded context to make a hunk fit, so check that the hunk landed where
+intended before accepting the refreshed result.
+
+## GCC's two queues
+
+`gcc` and `gcc-final` apply to the same GCC tree. `gcc` goes on first and
+affects both the bootstrap cross-compiler and the distributed compiler.
+`gcc-final` goes on top afterward, just before the distributed compiler
+is configured, and affects only that compiler. Patches in `gcc-final`
+therefore apply to an already-patched tree, and must be refreshed that
+way:
+
+    $ QUILT_PATCHES=$W/src/gcc quilt --quiltrc $W/src/quiltrc push -a --refresh
+    $ rm -rf .pc
+    $ QUILT_PATCHES=$W/src/gcc-final quilt --quiltrc $W/src/quiltrc push -a --refresh
+
+Removing `.pc` between queues mirrors the Dockerfile, which never pops.
+
+The w64devkit distribution includes this directory and `quilt.exe`, so
+the same commands work inside the kit.
+
+[quilt]: https://savannah.nongnu.org/projects/quilt

src/quiltrc

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+# Shared quilt settings for the patch queues under src/, so everyone
+# produces identical patches: quilt --quiltrc path/to/src/quiltrc ...
+# See src/README.md.
+QUILT_NO_DIFF_INDEX=1
+QUILT_NO_DIFF_TIMESTAMPS=1
+QUILT_REFRESH_ARGS="-p ab --diffstat"