Split the README reference material into docs/

The README was 794 lines: the summary, the design rationale, the whole macro
reference, the threading contract, the build internals and the exit-status
specification in one file. It is now 178 lines -- summary, installation,
quickstart, and an index -- and the reference material lives in docs/, one file
per topic: architecture, usage, status-codes, uncaught-errors, exit-status,
thread-safety, building.

The prose moved as written. Inbound references followed it: UPGRADING.md,
TODO.md, include/akerror.tmpl.h and tests/err_threads_handoff.c now name the
docs/ file that owns the text they cite, and AGENTS.md says where new
documentation goes so the README does not grow back.

Five factual errors fixed in the moved text:

- Both NULL-pointer examples inverted their test. FAIL_ZERO_* fails when the
  expression is zero, so `(somePointer == NULL)` failed on a *valid* pointer.
  They now read `(somePointer != NULL)`.
- AKERROR_NOIGNORE, four times including the #define, is AKERR_NOIGNORE.
- FINISH_NORExbTURN is FINISH_NORETURN.
- "functiions" is "functions".
- The architecture link pointed at include/akerror.h, which is generated and
  not in the tree; it points at include/akerror.tmpl.h.

The quickstart is new text. It compiles under -Wall -Wextra -Werror and was run
through all three of its paths: handled usage error exits 0, unhandled IO error
prints a trace and exits with the status, success exits 0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

TODO.md

@@ -60,7 +60,7 @@ overwrites that buffer in place, so a thread reading the name at that moment can
 see a torn string. Every other registry operation is serialized; this one cannot
 be, because the reader is outside the lock by the time it reads the characters.
 
-Documented in README.md and UPGRADING.md as "register names during
+Documented in docs/thread-safety.md and UPGRADING.md as "register names during
 initialization". Closing it properly means making a registered name immutable —
 either refusing a rename outright (a behavior change, and
 `tests/err_name_ownership.c` asserts the current contract), or copying names
@@ -114,11 +114,12 @@ behind a flag rather than in the default target or in CI.
 
 ## 9. No way to keep an error context and report it at the same time
 
-A context can be handed to another thread and released there -- `README.md` now
-documents that pattern and `tests/err_threads_handoff.c` proves it -- but it is a
-*move*. A thread that wants to both keep its error and report it upward has to
-read the fields out into its own record, and it loses the stack trace doing so,
-because `stacktracebuf` is the one thing that cannot be usefully summarized.
+A context can be handed to another thread and released there --
+`docs/thread-safety.md` now documents that pattern and
+`tests/err_threads_handoff.c` proves it -- but it is a *move*. A thread that
+wants to both keep its error and report it upward has to read the fields out
+into its own record, and it loses the stack trace doing so, because
+`stacktracebuf` is the one thing that cannot be usefully summarized.
 
 Copying the struct is not a workaround. `stacktracebufptr` is self-referential
 (`include/akerror.tmpl.h`), so `akerr_ErrorContext c = *src;` leaves the copy's
@@ -153,7 +154,8 @@ point -- `libakstdlib`'s planned `pthread_*` wrappers are the likely first.
 
 - The `AKERR_USE_STDLIB=OFF` build does not compile at all: `bool`, `PATH_MAX`
   and `NULL` are used unconditionally but only included under the stdlib branch.
-  The README's dependency list states what a replacement must provide, but the
-  header still needs its includes untangled for that configuration to work.
+  `docs/building.md`'s dependency list states what a replacement must provide,
+  but the header still needs its includes untangled for that configuration to
+  work.
 - `CMakeLists.txt` sets `main_lib_dest` from `MY_LIBRARY_VERSION`, which is never
   defined and never read. Dead line.