docs: describe Tier A parameterized classes work

Document grammar/pform changes, box_default smoke, and C#(byte) TODO on the iverilog-uvm track only.

docs/param-classes.md

@@ -0,0 +1,52 @@
+# Tier A #1 — Parameterized classes
+
+Track: **muhammadjawadkhan/iverilog-uvm** only (`feat/param-classes`). Do not open PRs to `steveicarus/iverilog` for this work.
+
+## Goal
+
+Support SystemVerilog parameterized classes needed for Accellera-shaped UVM (`uvm_*#(T)`, `config_db`, etc.):
+
+```systemverilog
+class C #(type T = int, parameter int W = 8);
+  // ...
+endclass
+
+C#(byte) c;   // explicit specialization — not done yet
+```
+
+## What landed (ANSI parameter ports + defaults)
+
+### Grammar (`parse.y`)
+
+`class_declaration` now pushes the class scope **before** the optional `#(...)` parameter port list (same idea as modules), then applies `extends`:
+
+1. `K_virtual_opt K_class lifetime_opt identifier_name` → start class / typedef (`pform_start_class_declaration` with base type deferred).
+2. `module_parameter_port_list_opt` — reuse the existing module ANSI parameter-port grammar so `type T = int`, `parameter int W = 8`, etc. land in the class `LexicalScope`.
+3. `class_declaration_extends_opt ';'` → `pform_set_class_extends(...)`.
+4. class items / `endclass` as before.
+
+### Pform (`pform_pclass.cc` / `pform.h`)
+
+- **`pform_set_class_extends`** — fills `base_type` / `base_args` on the current class after the parameter port list (replacing the old path that passed extends into `pform_start_class_declaration` before `#(...)` could exist).
+- Related: `pform_start_class_declaration`, `pform_end_class_declaration`, property helpers unchanged in role.
+
+With defaults present, a bare instance like `box b;` elaborates using those defaults.
+
+### Smoke example
+
+[`examples/param_classes/box_default.sv`](../examples/param_classes/box_default.sv) — `class box #(type T = int, parameter int W = 8);` constructed and used without an explicit `#()` override. Companion regression: `plain_class.sv`.
+
+```bash
+iverilog -g2012 -o box_default.vvp examples/param_classes/box_default.sv && vvp box_default.vvp
+# expect: PASS box_default ...
+```
+
+## TODO
+
+- [ ] Explicit specialization / overrides: `C#(byte)`, `C#(byte, 16)`, named overrides — parse, elaborate a specialized class type, and wire instances to it.
+- [ ] Inheritance + parameters interactions as needed by UVM base classes.
+- [ ] Broader ivtest coverage beyond the local smoke examples.
+
+## Status pointer
+
+See [STATUS.md](STATUS.md) (Parameterized classes row) and [ROADMAP.md](ROADMAP.md) Tier A item 1.