Install, CI, seeds and docs for the compiler written in Finch

bootstrap/install.sh and a share/finch lookup for the runtime and std; an ARM64 Linux seed; bootstrap/README.md;
CI builds from the seed on x64 and ARM64 and runs every test on Windows (Wine) and ARM64 (qemu); README, install and
layout sections of the guides (EN+PL) no longer talk about CMake; editors know enum, match and export.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015f73FesGZGYPkmDvUcj6QX

bootstrap/README.md

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+# The seed
+
+Finch's compiler is written in Finch. To build it you need something that already runs Finch: the **seed**, which is
+the compiler's own LLVM IR, one file per system:
+
+| File | System | Checked |
+|---|---|---|
+| `linux-x86_64.ll` | Linux x86-64 | built and tested here and in CI |
+| `linux-aarch64.ll` | Linux ARM64 | made with the ARM64 cross compiler; CI builds and tests it on an ARM64 runner |
+
+`bootstrap/build.sh` compiles the seed with `clang` (and the runtime, and libclang) into `build/finch0`; `finch0` builds
+`src/` into `finch1`; `finch1` builds `src/` again into `build/finch`; the two must make the same IR, or the script stops.
+
+## Why one per system
+
+The seed includes how this system's C compiler passes structs by value, because the compiler calls libclang with
+`CXCursor`, `CXType` and `CXString` (the calling convention is different on x86-64, on ARM64 and on Windows). So a seed
+made for one system doesn't run on another, and a seed needs the target system's C headers to be made.
+
+## When to make a new one
+
+When the compiler's own source starts to use a feature that the committed seed doesn't have, `build.sh` fails at the first
+step. Make the seed from a working compiler:
+
+```sh
+bootstrap/build.sh seed            # writes bootstrap/<this system>.ll
+```
+
+For another system, from a machine that has that system's headers and a cross compiler (what `tests/cross.sh` uses):
+
+```sh
+FINCH_CC=aarch64-linux-gnu-gcc build/finch ir src/main.fch --target arm64 > bootstrap/linux-aarch64.ll
+```
+
+## macOS and Windows
+
+There is no seed for them yet: it needs Apple's or Microsoft's headers, which can't be had on Linux. Until there is one,
+programs for macOS and Windows are built from Linux with `--target macos|windows`, and the compiler's older C++ version
+(branch `before-bootstrap`) is the way to get a compiler that runs there.

bootstrap/install.sh

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+#!/bin/sh
+# Installs the compiler you built (bootstrap/build.sh) under a prefix:
+#   bootstrap/install.sh               /usr/local   (needs sudo)
+#   bootstrap/install.sh ~/.local
+# finch goes to <prefix>/bin; the runtime and the standard library go to <prefix>/share/finch, where finch looks for them.
+set -eu
+cd "$(dirname "$0")/.."
+prefix=${1:-/usr/local}
+[ -x build/finch ] || { echo "install: run bootstrap/build.sh first" >&2; exit 1; }
+mkdir -p "$prefix/bin" "$prefix/share/finch"
+cp build/finch "$prefix/bin/finch"
+rm -rf "$prefix/share/finch/runtime" "$prefix/share/finch/std"
+cp -r runtime std "$prefix/share/finch/"
+echo "installed: $prefix/bin/finch ($("$prefix/bin/finch" version))"