Signex v0.13.0

The v0.13 Symbol & Library milestone. This release pairs the
cleanroom schematic renderer (the work planned as v0.12) with a wave of
symbol-editor and library polish. There is no separate v0.12.0 tag — the
cleanroom rewrite ships here. The workspace version had already advanced
to 0.13.0, so this release adopts that number rather than bumping
backward.

Headline — cleanroom schematic renderer

• Clean-room reimplementation of crates/signex-renderer/src/schematic.rs
  (label / symbol / field-style rendering) and the field-autoplace
  heuristic, executed against Signex-only specifications
  (docs/RENDERING_RULES.md, Altium parity goals, IEEE-Std-91) rather
  than any third-party EDA source. Schematic rendering output changes
  subtly versus v0.11 — label placement, field rotation/justification,
  and IEEE-Std-91 pin decorators are now driven by the documented rules.
  This is the milestone tracked as "v0.12" in prior READMEs.

Added — symbol editor

• Unified active-bar widget. The .snxsym editor adopts the generic
  signex_widgets::active_bar in a single-call form, so the symbol
  editor's floating toolbar matches the schematic editor byte-for-byte
  (root highlight, dropdown panels, right-click, chevron). New
  active_bar_dropdowns module backs the per-tool dropdown overlays;
  dropdown panels position relative to the bar's y_offset so they open
  directly under their trigger button.
• .snxsym TOML+TSV envelope. Standalone symbol files serialise to
  the same TOML-header + TSV-bulk envelope as the rest of the Signex
  format family (SymbolFile::to_toml_string / from_bytes), so pin
  tables are line-diffable in git. Legacy JSON .snxsym files still load
  (auto-detected on open).

Added — library

• Library-subsystem polish across the Library Browser, Component
  Preview, and standalone primitive tabs: inline Pick Symbol / Pick
  Footprint binding with read-only cells, project-tree listing of
  .snxsym / .snxfpt files (rather than individual primitives),
  Save-As flow for standalone primitive libraries, and assorted binding
  / refresh fixes.

Added — per-file history

• Per-file Git history right-dock panel refinements — the History
  panel follows the active tab and renders the file's recent commits via
  signex_widgets::history_pane, async-loaded with a generation counter
  to drop stale results on tab switch.

Changed — footprint editor hidden for this release

• The footprint / sketch editor is gated off in v0.13.0. It is
  feature-incomplete and was under heavy daily iteration; rather than
  ship an unfinished editor, its user-facing entry points are disabled
  behind a compile-time flag (signex_app::feature_flags::FOOTPRINT_EDITOR_ENABLED).
  Opening a .snxfpt no longer pushes an editable Footprint Editor tab,
  and the "New Footprint / PCB Library" create flow is removed from the
  command palette and project-tree menus.
• Footprints remain first-class data. Read-only footprint preview in
  the Component Preview tab, Pick Footprint binding of existing .snxfpt
  files into component rows, the footprint column in the Library Browser,
  and the bake / library backend are all unchanged. Only the editor
  surface is hidden. The full editor returns in a later release by
  flipping the flag.

Tests

• New regression coverage pinning the footprint gate
  (opening_snxfpt_does_not_create_editable_tab_when_gated) plus a
  positive control proving the symbol editor still opens
  (opening_snxsym_still_creates_editable_tab). Full cargo test --workspace green.

Constraints — Apache-clean invariants (carry forward from v0.9.0)

• Zero kicad/KiCad/KICAD substrings under crates/; no
  kicad-parser / kicad-writer deps or imports; no removed-API
  surface re-introduced; cargo-deny check licenses green. The
  cleanroom renderer was authored against Signex-only specs with no
  third-party EDA source in context.