Replace IR snapshots with a --dot graph writer

[jserv]

The IR snapshot suite pinned frontend IR shape for three programs across two architectures and two link modes, and cost two shell scripts, twelve checked-in JSON files, four Makefile targets that reconfigured the tree as a side effect, a CI job, a graphviz and jq prerequisite gate on every build, and a sed and jq pipeline whose only job was to launder unstable output. This replaces that machinery with a --dot flag that writes the SSA control flow graph as Graphviz DOT and stops, and moves the three programs into tests/driver.sh, where they now also run under the self-hosted compiler; the Makefile rule they replace only ever built them with stage 0.

Making --dot work self-hosted turned up a wrong-code bug worth calling out separately. optimize() rewrote the constant operand of a power-of-2 multiply, divide or modulo in place, and rename_var() gives every use reached by one definition the same var_t, so the rewrite changed the value every other use saw and compounded. int k = 8; return a * k + b * k; returned 25 rather than 40, on the host built and self-hosted compilers alike. It is the first commit here, isolated so it can be read and backported on its own.

The rest is what the graph writer needed and what looking closely turned up: %p in the embedded libc, whose formatter silently emitted nothing for an unmatched conversion; a bounded staging buffer shared by printf and fprintf, which previously wrote past a 200 byte frame; bounds on several frontend buffers that could be overrun from ordinary input; diagnostics with a file, line and column where an undeclared name or a missing ternary colon used to fault or abort; and a split of x64-codegen.c into an encoder and an IR walk, matching how arm.c relates to arm-codegen.c.

Verified on arm, riscv and x64, plus arm with dynamic linking: make check passes 609/609 at stage 0 and stage 2 on each, the ABI suites pass, and stage 1 and stage 2 are byte-identical everywhere. .ci/check-format.sh and .ci/check-newline.sh are clean. Every commit builds, so the series bisects. Self-hosted --dot on tests/fib.c now produces the same 3470 line graph as the host build with no colliding node ids, where before the fix it collapsed 2234 nodes onto one.

Deliberately not included: nothing now checks IR shape, so a frontend change that alters the graph while the programs still print the same thing goes unnoticed. --dot output is deterministic apart from the pointer values in node names, so a normalized diff against a checked-in reference would restore that cheaply, without graphviz or jq, but that belongs in its own change.

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Summary by cubic

Replaces the IR snapshot suite with a --dot flag that writes the SSA control flow graph as Graphviz DOT and exits, and moves the three snapshot programs into tests/driver.sh, where they now run under both the host and self-hosted compilers. This deletes twelve canned JSON files, two shell scripts, a CI job, and the graphviz/jq prerequisite from every build.

• Fixes a wrong-code bug where strength reduction rewrote a shared constant in place, causing a * k + b * k to compute 25 instead of 40.
• Bounds frontend buffers that ordinary input could overrun: long numeric and character literals are now accepted instead of rejected, and output past the staging buffer is no longer cut.
• %p prints a whole pointer on LP64 instead of truncating it, and now renders the same way whether or not the pointer has a high word.
• Refuses a --dot run whose output name would overwrite its input, whether derived from a .dot input or given explicitly with -o.
• Splits x86-64 instruction encoding into a new src/x64.c, matching how arm.c relates to arm-codegen.c, and reports user errors with file, line, and column instead of aborting.
• driver.sh now surfaces the compiler's own diagnostics, reports every failure instead of stopping at the first, honors TEST_FILTER and FAIL_FAST, and removes its temp directory unless something failed.
• Peephole removals now keep each block's IR tail valid, dropping the x64 backend's workaround walk.
• Drops dead declarations and raises MAX_IR_INSTR to 262144, since a self-compile emits about 101k.

Note: nothing now asserts on IR shape, so a frontend change that alters the graph while output stays identical would go unnoticed.

^{Written for commit a8dc9e5. Summary will update on new commits.}