feat(aig): make the vendored-PDK root configurable via JACQUARD_VENDOR_DIR

[robtaylor]

Problem

The gf180mcu / sky130 cell-library decomposition in AIG::from_netlistdb_* (src/aig.rs) hardcodes the PDK functional-model path to vendor/..., resolved relative to the process CWD:

let pdk_path = std::path::PathBuf::from("vendor/sky130_fd_sc_hd/cells");
...
let pdk_path = std::path::PathBuf::from("vendor/gf180mcu_fd_sc_mcu7t5v0");

This works when you run jacquard from the repo root (where the vendor/ submodule lives). But a consumer that embeds jacquard as a submodule and invokes sim/cosim from the parent repo's working directory has no way to point at the vendored PDKs — the only workaround today is to symlink vendor/ into the parent CWD.

Fix

Resolve the vendor root through a small helper that honours a new JACQUARD_VENDOR_DIR environment variable and defaults to vendor:

fn pdk_vendor_root() -> PathBuf {
    env::var_os("JACQUARD_VENDOR_DIR").map(PathBuf::from)
        .unwrap_or_else(|| PathBuf::from("vendor"))
}

• Backward-compatible — unset behaviour is byte-identical to before (vendor/...).
• Follows the existing JACQUARD_* runtime-knob convention (JACQUARD_SRAM_DUMP, etc.).
• Downstream callers set JACQUARD_VENDOR_DIR=path/to/jacquard/vendor instead of symlinking.
• The gf180 "PDK models required" panic hint now mentions the env var.

Tests

The env-independent resolution logic is split into a pure pdk_vendor_root_from(Option<OsString>) and unit-tested (default + override). cargo test --lib pdk_vendor_root is green.

A CLI flag could later layer on top of the same helper if preferred; this PR keeps the change minimal and matches the established env-var pattern.