docs: how to use the 1990 r5.01 (OS/2) TMS34010 toolchain

The r5.01 tools are a superset of the 1987 DOS set: ANSI C, one driver,
gspopt, a bigger runtime, and 35.8% smaller .text at -o2 on the same
source. Add a practical guide:

* what the package contains and how to extract it
* running it: OS/2 1.3 under 86Box (full toolchain) and 2ine (assembler/
  linker/converter, with the patched fork linked)
* the gspcl option table and verified command lines (-o0/-o1/-o2, -pk,
  -al, -z/-lrts.lib), plus the manual gspac -> gspopt -> gspcg -> gspa
  pipeline and the linker command file
* gotchas: -o semantics, bit-addressed section sizes, CRLF, AUTOEXEC not
  running in the guest, emulated-keyboard limits, one-way assembler
  compatibility
* measured code-size and throughput numbers for both toolchains

Link it from the docs index and the repository layout.

README.md

@@ -568,6 +568,7 @@ also why a server-side change cannot cause or cure it.
 | `reference/level1-test-sequence.md` | the 15 level-1 tests and their codes |
 | `reference/third-party-code.md` | what open-source code is in the ROM |
 | `reference/tms34010-sdk.md` | the 1987 toolchain, buildable today |
+| `tools/tms34010-sdk/OS2-501.md` | using the 1990 r5.01 toolchain (ANSI C + optimizer) |
 | `EXPLOIT_NOTES.md` | the original code-execution audit and the crash decode |
 | `NEXT_SESSION.md` | the LAT soak handoff (older; see the reports for later work) |
 
@@ -587,7 +588,7 @@ vt1200/
 │   ├── ptr_constants.py         # find/resolve bit-address constants
 │   ├── smart34010/              # dis34 (two encoding fixes)
 │   ├── as34/                    # TMS34010 binutils, ported to 64-bit Linux
-│   ├── tms34010-sdk/            # the 1987 TI SDK (under DOSBox)
+│   ├── tms34010-sdk/            # TI SDKs: 1987 DOS + 1990 OS/2 (see OS2-501.md)
 │   └── ghidra-tms34010/         # patched processor module + install script
 ├── harness/
 │   ├── mame/                    # the MAME driver, Lua probes, reports