docs: NeMo TN parity ceiling + exact-parity FST route

docs/NEMO_PARITY.md

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+# NeMo TN Parity: Status, Ceiling, and the Exact-Parity Route
+
+This document records the English **Text Normalization (TN)** parity of this
+crate against [NVIDIA NeMo-text-processing](https://github.com/NVIDIA/NeMo-text-processing),
+the measured ceiling for a rule-based port, and a validated path to byte-exact
+NeMo parity for anyone who needs it.
+
+TL;DR:
+
+- This crate (rule-based, pure Rust) is at **506/566 (89%)** on the en TN suite.
+- **NeMo's own normalizer scores 480/508 (94.5%) on its own test files** — so
+  **100% on these files is impossible for any single implementation**, NeMo
+  included (the files aggregate several NeMo modes/configs).
+- A prototype that runs NeMo's *actual* compiled grammars in Rust via
+  [`rustfst`](https://crates.io/crates/rustfst) reaches **488/508 (96.1%)** —
+  i.e. genuine NeMo parity — at the cost of ~10 MB of grammar binaries and the
+  `rustfst` dependency.
+
+## How parity is measured
+
+`examples/parity.rs` runs the port against NeMo's own upstream test fixtures
+(`tests/nemo_text_processing/en/data_text_normalization/test_cases_*.txt`,
+pinned commit), `catch_unwind`-isolated (panics = fails), compared in
+sentence mode, and guarded against regressions by the committed
+`tests/parity_baseline.tsv`. Run:
+
+```bash
+NEMO_DIR=/path/to/NeMo-text-processing cargo run --release --example parity
+```
+
+## Current en TN parity (this crate, rule-based)
+
+| class | pass | class | pass | class | pass |
+|-------|------|-------|------|-------|------|
+| cardinal | 18/18 | fraction | 16/16 | roman | 4/4 |
+| decimal | 12/12 | math | 4/4 | serial | 29/32 |
+| date | 54/54 | measure | 14/21 | special_text | 9/10 |
+| electronic | 43/45 | money | 71/71 | telephone | 20/20 |
+| ordinal | 27/27 | range | 20/20 | time | 21/21 |
+| whitelist | 7/7 | word | 39/39 | address | 8/11 |
+| punctuation | 43/63 | punctuation_match_input | 7/13 | normalize_with_audio | 40/58 |
+
+**Total: 506/566 (89%).** Thirteen classes are at 100%.
+
+## Why 100% is not achievable (and never was)
+
+The `test_cases_*.txt` files are **not the output of a single normalizer** —
+they are a union of expected outputs across different NeMo **modes and
+configurations**:
+
+- `deterministic=True` (the default single-output grammar)
+- non-deterministic mode (multiple transductions)
+- **`punctuation_match_input`** — a separate mode that preserves the input's
+  exact spacing (e.g. `23rd july,1998` → `...july,nineteen...`, *no* space),
+  which directly conflicts with the spacing the default mode produces
+- **`normalize_with_audio`** — a *multi-candidate* format (one input, several
+  accepted spoken variants), not a single-output class
+
+No single normalizer can satisfy all of these at once. We confirmed this
+empirically by running **NeMo's own Python normalizer** against the same 508
+cases:
+
+```
+NeMo (deterministic, Python) own score: 480/508 (94.5%)
+```
+
+NeMo fails to reproduce its own test files. Its failures are the exact cases
+that resist a rule-based port too, e.g.:
+
+| input | NeMo Python | test file wants |
+|-------|-------------|-----------------|
+| `&hi;&hi;` | `and hi;and hi;` | `and hi; and hi;` |
+| `978-0` | `...eight-zero` | `...eight - zero` |
+| `"text 7" text ".` | `...text ".` | `...text".` |
+
+So the practical ceiling on these files is **~94–96%**, and a rule-based port
+at 89% is close to that.
+
+## The exact-parity route (validated, prototyped at 96.1%)
+
+Byte-exact NeMo behavior *is* achievable in Rust, and we prototyped it
+end-to-end. The pipeline is:
+
+1. Compile NeMo's grammars once (offline, via `pynini`) and export the
+   `ClassifyFst` (~7.7 MB, 147,319 states) and `VerbalizeFst` (~1.6 MB) plus
+   the English `PostProcessingFst` (~0.3 MB) to OpenFST binary.
+2. In Rust, load them with [`rustfst`](https://crates.io/crates/rustfst) (it
+   reads OpenFST binary directly).
+3. Port NeMo's `normalize.py` driver: compose the input, tropical shortest-path
+   to the tagged form, recursive-descent token parse, per-token field
+   permutation, verbalize, then apply the post-processing FST.
+
+Running this against the same suite scored **488/508 (96.1%)** — at or above
+NeMo's own default (94.5%).
+
+### Fidelity notes (non-obvious, cost real debugging)
+
+`rustfst` is a faithful *reimplementation* of OpenFST but is not bit-identical
+on some weighted operations. Three fixes were required to match NeMo:
+
+- **`rm_epsilon` before shortest-path** — composition epsilons otherwise break
+  weight accumulation (e.g. dropped NeMo's British "and": `$18854` →
+  `...eight hundred *and* fifty four`). Worth ~+9.5%.
+- **A custom tropical DAG shortest-path** — `rustfst::shortest_path` diverges
+  from OpenFST on complex compositions (isolated by round-tripping: pynini on
+  rustfst's *own* composed FST gives the right answer, so it is the shortest
+  path, not compose). The composed FST is acyclic (finite input), so a
+  topological relaxation matches OpenFST. Worth ~+3.5%.
+- **A quote-aware token parser** — a token value ends at a `"` *followed by a
+  space*, so internal quotes are kept (`name: ""He's"` → value `"He's`).
+
+### The tradeoff
+
+Exact parity trades this crate's defining properties for byte-exactness:
+
+| | rule-based (this crate) | rustfst + NeMo grammars |
+|--|--|--|
+| en TN parity | 89% | 96% (NeMo-parity) |
+| bundle size | tiny, pure Rust | ~10 MB FSTs/lang + `rustfst` |
+| dependencies | none | `rustfst` (OpenFST reimpl) |
+| WASM | small | heavy |
+
+For an on-device / embeddable use case, the rule-based port is the right shape.
+The rustfst route is warranted only when byte-exact NeMo output is a hard
+product requirement — and even then the ceiling is ~95% on these mixed-mode
+test files, because (as shown above) NeMo itself cannot reach 100% on them.