docs: run the r5.01 C toolchain end to end under 2ine

The null-selector fix in the 2ine fork unblocks gspac/gspopt/gspcg, so
document the full pipeline: build 2ine, extract the tools, the four C
stages, absolute placement, and the firmware round-trip.

Record the two gotchas found while testing: gspcl still stops after
gspac because the children exit 139 at teardown, and GSPA 5.01 rejects
b15, so the 32 KB round-trip differs at one SP access (same physical
register, A15/B15).

README.md

@@ -572,6 +572,7 @@ also why a server-side change cannot cause or cure it.
 | `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) |
+| `tools/tms34010-sdk/2INE-501.md` | the r5.01 C toolchain end to end under 2ine, no VM |
 | `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) |
 
@@ -591,7 +592,7 @@ vt1200/
 │   ├── ptr_constants.py         # find/resolve bit-address constants
 │   ├── smart34010/              # dis34 (two encoding fixes)
 │   ├── as34/                    # TMS34010 binutils, ported to 64-bit Linux
-│   ├── tms34010-sdk/            # TI SDKs: 1987 DOS + 1990 OS/2 (see OS2-501.md)
+│   ├── tms34010-sdk/            # TI SDKs: 1987 DOS + 1990 OS/2 (see OS2-501.md, 2INE-501.md)
 │   └── ghidra-tms34010/         # patched processor module + install script
 ├── harness/
 │   ├── mame/                    # the MAME driver, Lua probes, reports