Add package targets, build profiles, and unified artifact layouts

[LunaStev]

Summary

Add manifest-owned package targets, build profiles, and one artifact layout for ordinary and native builds. A package can now expose its canonical Wave library alongside multiple named binaries, and Vex validates every selected build plan before compiling the first target.

Closes #46
Closes #47
Closes #48
Closes #54

Why

The previous single lib switch could not represent a library plus CLI or multiple binaries. Ordinary builds delegated artifact placement to wavec while native builds used a separate target/profile layout, and --release was only a hard-coded optimization flag. These differences prevented a consistent package build contract and reliable tooling output.

Behavior

• Manifest format 3 declares one optional targets.lib, explicit targets.bins, default_run, profiles, a default compiler target, and exact-triple platform settings. Formats 1 and 2 retain their parsing/string meanings and are not rewritten automatically. Lockfile v3 and locked/offline graph semantics remain unchanged.
• build/check default to all root targets, with --lib and repeatable --bin <name> selectors. run chooses an explicit binary, default_run, or the only binary; ambiguity is an error. Dependencies contribute their source library, not their binaries or separately emitted library objects.
• Profiles support dev/release and custom profiles inheriting one builtin, with optimization levels 0–3. --release aliases --profile release. Debug information, overflow/assertion switches, LTO, and size-optimization policies without a guaranteed compiler contract are rejected rather than silently ignored. Wave language entry/main semantics remain compiler-owned.
• All manifests now emit into target/<compiler-triple>/<profile>/bin/ or lib/, with separate intermediates and protected Windows companion files. Old artifacts are not automatically moved or deleted. Compiler plans must honor the selected output kind, final destination, and intermediate namespace.
• Hosted, freestanding ELF, WASI and bare Wasm configurations use typed mode/entry/startup/linker-script settings. Root-owned linker/sysroot settings do not propagate from dependencies. Unsupported compiler combinations fail explicitly; external SDK contents are not pinned by the Wave dependency lockfile.
• Every selected target is planned and checked for output collisions/ownership before compilation. Execution stops at the first failure and keeps earlier successful artifacts. Run generations and their leases remain isolated; the project build guard is released before spawning the program.
• Metadata adds --format-version 2 for target/profile/platform declarations without compiler queries. The default remains schema 1, which rejects packages it cannot represent accurately. Doctor checks every root target.
• Dry-run stdout changes for all manifests: it is now one versioned vex.build-plan envelope containing the selected profile and per-target compiler plans. Each selected target invokes the planner once; dry-run creates no build outputs or message file.

The migration and command contracts are documented in docs/targets-and-profiles.md, the reference pages, and JSON schemas. Existing required real-compiler CI smokes now exercise format 3 mixed targets, self-imports, profiles, metadata and native exports. Contributor good first issue tasks are not included or linked for automatic closure.

Validation

• cargo fmt --check
• cargo test --workspace --locked — 289 passed; one existing subprocess-fixture test is intentionally ignored and invoked by supervision tests.
• cargo clippy --workspace --locked --all-targets -- -D warnings
• cargo build --workspace --locked
• python3 x.py check — includes the checks above and 38 Python tests.
• Parser mutation corpus: 20,000 cases, seed 20261018, including the new format 3 model.
• CLI reference, Markdown links/anchors, JSON schema parsing and repository platform-contract checks.
• Workspace cross checks: x86_64-pc-windows-gnu, aarch64-unknown-linux-gnu, riscv64gc-unknown-linux-gnu.
• Linux real-compiler wave_compatibility.py and native_interop.py, including persisted format 3 cases, C/C++/Rust inputs and C consumers of Wave object/shared artifacts.
• Additional real-compiler build/check of freestanding ELF, WASI, bare wasm32/wasm64, and mixed library/binary Wasm artifacts.

The local compiler reports 0.2.1-pre-beta; its executable SHA-256 is e4439dc77b0a142dd38e22406cf489c2ee123bcf2ec474afbefd072078a0e504. Its exact build source commit was not inferred from its version. Cross checks establish compilation, not native runtime acceptance; all nine platform CI lanes must pass on this PR before merge.

Checklist

• The change is focused and targets wavefnd/Vex:master.
• User-facing behavior and recovery instructions are documented.
• Dependency changes preserve lockfile reproducibility.
• --locked and --offline behavior remains correct.
• No raw wavec flags were added to the Vex CLI.
• New commits include a DCO Signed-off-by: line.