tools/sgpiom: add README for JSON tooling workflow

tools/sgpiom/README.md

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+# SGPIOM JSON Tooling
+
+This directory contains AST10x0 SGPIOM configuration tooling that replaces DTS-style wiring with a declarative JSON pipeline.
+
+## What This Builds
+
+The tool consumes board manifests and produces deterministic build artifacts:
+
+- `sgpiom_merged.json`: canonical merged manifest
+- `sgpiom_config_generated.rs`: generated Rust config module
+- `sgpiom_report.txt`: summary report (optional)
+- check/validate stamp outputs for CI gates
+
+Generated Rust symbols include:
+
+- `SGPIOM_CONTROLLER`
+- `SGPIOM_BANKS`
+- `SGPIOM_SIGNALS`
+- `SGPIOM_MANIFEST_HASH`
+
+## CLI
+
+Tool entrypoint:
+
+- `tools/sgpiom/sgpio_json_tool.py`
+
+Subcommands:
+
+- `validate`: schema and semantic checks
+- `merge`: deterministic merge of input manifests
+- `generate`: emit Rust static config module
+- `check`: verify generated output is up to date
+- `report`: print human-readable usage summary
+
+## Example Inputs
+
+- `tools/sgpiom/examples/common.json`
+- `tools/sgpiom/examples/ast1060_dcscm.json`
+- `tools/sgpiom/examples/ast1060_prot_dice.json`
+
+## Local Usage
+
+Validate:
+
+```bash
+python3 tools/sgpiom/sgpio_json_tool.py validate \
+  --input tools/sgpiom/examples/common.json \
+  --input tools/sgpiom/examples/ast1060_dcscm.json
+```
+
+Merge:
+
+```bash
+python3 tools/sgpiom/sgpio_json_tool.py merge \
+  --input tools/sgpiom/examples/common.json \
+  --input tools/sgpiom/examples/ast1060_dcscm.json \
+  --output /tmp/sgpiom_merged.json
+```
+
+Generate:
+
+```bash
+python3 tools/sgpiom/sgpio_json_tool.py generate \
+  --input tools/sgpiom/examples/common.json \
+  --input tools/sgpiom/examples/ast1060_dcscm.json \
+  --output /tmp/sgpiom_config_generated.rs
+```
+
+Check:
+
+```bash
+python3 tools/sgpiom/sgpio_json_tool.py check \
+  --input tools/sgpiom/examples/common.json \
+  --input tools/sgpiom/examples/ast1060_dcscm.json \
+  --output /tmp/sgpiom_config_generated.rs
+```
+
+Report:
+
+```bash
+python3 tools/sgpiom/sgpio_json_tool.py report \
+  --input tools/sgpiom/examples/common.json \
+  --input tools/sgpiom/examples/ast1060_dcscm.json
+```
+
+## Bazel Integration
+
+Bazel tool package:
+
+- `//tools/sgpiom`
+
+Board-level SGPIOM pipeline targets:
+
+- `//target/ast10x0/board:sgpiom_validate`
+- `//target/ast10x0/board:sgpiom_merged`
+- `//target/ast10x0/board:sgpiom_generate`
+- `//target/ast10x0/board:sgpiom_check`
+- `//target/ast10x0/board:sgpiom_report`
+
+Board crate consumption target:
+
+- `//target/ast10x0/board:ast10x0_board`
+
+Build all SGPIOM pipeline targets:
+
+```bash
+bazelisk build --config=k_ast1060_evb \
+  //target/ast10x0/board:ast10x0_board \
+  //target/ast10x0/board:sgpiom_validate \
+  //target/ast10x0/board:sgpiom_merged \
+  //target/ast10x0/board:sgpiom_generate \
+  //target/ast10x0/board:sgpiom_check \
+  //target/ast10x0/board:sgpiom_report
+```
+
+## Notes
+
+- Board crate selects configuration; peripherals crate consumes typed config.
+- Keep manifests declarative and deterministic to avoid runtime parser complexity.