Rewrite backend: single-pass fasm pipeline (drops qbe/as/ld.lld)

[Prestgg1]

Summary

Replaces the whole code-generation/linking chain (qbe → .il → as → .o → ld.lld) with a single-pass fasm pipeline: the compiler emits one .asm file, and fasm assembles and links it directly into a standalone ELF. No object files, no separate linker, no LLVM/QBE toolchain — the only external dependency is fasm.

Before	After
qbe + as + ld.lld	fasm (one invocation)
.il → .s → .o → binary	.asm → binary
@link = prebuilt .o files + sdk/build.sh	@link = fasm source include (no build step)

What's changed

Backend

• crates/transpiler: full fasm emitter — stack-slot frame model (16-byte aligned), System V AMD64 call ABI (≤6 args in registers, overflow on stack), azfn_ user-function labels (avoids fasm reserved-word collisions like add), string data dedupe, fasm local branch labels, escape sequences (\\n, \\t, \\r, \\0, \\\\) resolved at emission
• validator: link_files is now (operation, source path); only operations actually called get their source included, each path exactly once
• compiler: builds a single .asm (header → filtered includes → code → data) and runs fasm; BackendError::Fasm surfaces fasm stderr
• sdk: assembly stubs are fasm syntax, lib.az @link → ./sdk/src/*.s, build.sh removed

Bugs fixed along the way (found by e2e)

• tokenizer: &&/|| caused an infinite loop in read_operator; =/! peek guard ate the first char of the next token (!flag → lag)
• parser: f() + g() / return f() + g() fell through the call path; loop bodies were never parsed
• validator: loop bodies were never validated
• string escapes: \\n etc. never worked in data emission (old QBE path handled it)

Tests
The suite did not compile at main HEAD (38 errors, pre-existing — verified against a clean worktree). Repaired the validator/tokenizer tests against the current API; all 18 test suites are green now.

Tooling & docs

• CI: installs fasm, checks out submodules
• .gitignore: *.ssa, *.asm, /sdk/ anchoring (azlang-docs/src/sdk/*.md was accidentally ignored), book output; tracked compiler binaries/.s/.ssa artifacts removed from git
• docs: qbe.md → fasm.md, assembly_linking.md → direct_assembly.md, all @link examples → ./sdk/src/*.s, diagrams/pipeline rewritten, mdbook build passes

Measured (not estimates)

Program	Size
examples/hello_world.az	198 bytes
Docs hello world (own @link, escapes)	228 bytes
60-line multi-module integration program	2113 bytes

file: ELF 64-bit LSB executable, x86-64, statically linked, no section header — 1 program header, no dynamic interpreter.

⚠️ Merge order

Companion PR for the SDK submodule must merge first: AzLangProgramingLanguage/AzLang-Std#1 (this PR's sdk gitlink points at 273f2b0 on its fasm-sdk branch).

Testing

• cargo test --workspace — 18 suites, all green
• azcli build + run for: hello world (198B), docs example (228B), integration program covering modules, recursion, while/loop/break/continue, if/elif/else, div/mod, &&, string ==, exit — all assertions pass, exit 0
• mdbook build passes

[Prestgg1]

it looks great but I can't merge this pr. Because it's vibe coded