feature: add mkdocs-material documentation site with CI deployment

Generate comprehensive documentation for all 173 source units and
integrate mkdocs-material into the build toolchain. Documentation
covers kernel core, x86 architecture, drivers, applications, and
all subsystems with a tabbed navigation structure.

- Add mkdocs.yml with material theme, mermaid support, and tabbed nav
- Add doc/index.md landing page
- Generate doc/src/ markdown for every Pascal unit in src/
- Reorganize planning docs into doc/planning/
- Add compat, lvglh, and toolchain doc sections
- Update Dockerfile to install python3 and mkdocs-material (pinned <2)
- Replace pasdoc-based compile_docs.sh with mkdocs build --strict
- Add compile_docs.sh to the main build pipeline in compile.sh
- Add deploy-docs step in .drone.yml with host volume mount to nginx
- Add site/.gitkeep for build output directory

doc/src/debug/debug.tracer.md

@@ -0,0 +1,103 @@
+# debug.tracer
+
+Ring buffer trace log for recording and inspecting recent kernel call paths.
+
+## Overview
+
+`debug.tracer` maintains a fixed-size circular array of `pchar` pointers representing the most recently pushed trace labels. Callers push a static string literal before a significant operation and pop it afterwards. Because no string copy is made, pointers must reference persistent memory (string literals or static constants).
+
+The tracer is disabled immediately at kernel startup by `freeze` and re-enabled by `init` once the infrastructure is stable enough to accept trace entries. A compile-time `TRACER_ENABLE` conditional controls whether the tracing code is compiled at all; if disabled, all calls become no-ops.
+
+A `TRACER` shell command allows runtime inspection and toggling of the trace buffer.
+
+## Dependencies
+
+- `core.util`, `arch.x86.util`
+- `core.strings`
+- `io.stdio`
+- `io.syslog`
+
+## Constants
+
+### MAX_TRACE
+`40` — Capacity of the trace ring buffer. At most 40 labels are retained; older entries are silently overwritten.
+
+## Functions and Procedures
+
+### init
+
+```pascal
+procedure init;
+```
+
+Zeros all trace slots, resets the `Locked` and `head` state variables, sets the tracer to ready, pushes an initial `'kmain'` entry, and registers the `TRACER` shell command via `io.stdio.registerCommand`.
+
+### push_trace
+
+```pascal
+procedure push_trace(t_name: pchar);
+```
+
+Advances `head` (with wrap-around at `MAX_TRACE`) and stores `t_name` at the new position. Uses a simple boolean `Locked` flag to prevent re-entrant writes. Does nothing if the tracer is frozen or `TRACER_ENABLE` is false.
+
+The pointer is stored directly; **no copy is made**. Callers must pass pointers to static/persistent data.
+
+### pop_trace
+
+```pascal
+procedure pop_trace;
+```
+
+Currently a no-op stub. Present for symmetry with `push_trace` and potential future implementation of a proper call-depth model.
+
+### freeze
+
+```pascal
+procedure freeze;
+```
+
+Sets the tracer to not-ready, preventing further entries until `init` is called. Called by `kmain` at the very beginning of the boot sequence before the heap is available.
+
+### get_last_trace
+
+```pascal
+function get_last_trace: pchar;
+```
+
+Returns the pointer stored at the current `head` position — the most recently pushed trace label.
+
+### get_trace_count
+
+```pascal
+function get_trace_count: uint32;
+```
+
+Returns `MAX_TRACE` (the ring buffer capacity). Does not reflect the number of distinct pushes made.
+
+### get_trace_N
+
+```pascal
+function get_trace_N(idx: uint32): pchar;
+```
+
+Returns the trace entry `idx` positions before the current head (0 = most recent). Handles ring-buffer wrap-around. Returns `nil` if `idx` is out of range or `TRACER_ENABLE` is false.
+
+### print_traces
+
+```pascal
+procedure print_traces;
+```
+
+Iterates the entire ring buffer from index 0 to `MAX_TRACE - 1` and writes each entry to `io.syslog` in the format `[TRACER] [N] <label>`. Entries that are `nil` are printed as `?????????`.
+
+## Notes
+
+The `TRACER` shell command supports three sub-commands:
+
+| Sub-command | Description |
+|---|---|
+| `list [count]` | Prints the last `count` (default 5, max 39) trace entries to stdout. |
+| `disable` | Sets the tracer to not-ready. Only works if `TRACER_ENABLE` is true. |
+| `enable` | Sets the tracer to ready. Only works if `TRACER_ENABLE` is true. |
+
+The `Locked` flag is a simple spin-lock that prevents nested `push_trace` calls from corrupting the ring buffer. It does not protect against concurrent ISR-level pushes; the tracer is not interrupt-safe by design.