Unpickling: share one EntityRef per non-local reference row

[auduchinok]

u_tcref wrapped a fresh EntityRef around the shared NonLocalEntityRef on every mention. Reading a
489-reference project that is 88642 wrappers around 1919 rows — 46 per row.

They are all equal. An EntityRef is the row plus a binding field that caches the resolution, and the
resolution is a function of the row, so sharing one wrapper per row also resolves once instead of once per
mention. The cache is an array beside the inlerefs table, keyed by the row index that is already being
read, so there is no lookup and no hashing.

project	base	change	delta
FSharp.Common	141.2	138.5	−2.68 (−1.9%)
Fantomas.Core.Tests	75.7	73.9	−1.77 (−2.3%)
Fantomas.Benchmarks	52.4	50.7	−1.73 (−3.3%)
Fantomas.Core	57.1	55.7	−1.37 (−2.4%)
FSharp.Compiler.Service	359.4	358.4	−1.03 (−0.3%)
Oxpecker	50.4	49.9	−0.54 (−1.1%)
consoleapp	28.3	27.9	−0.42 (−1.5%)
IcedTasks	26.3	25.9	−0.38 (−1.4%)
FsToolkit.ErrorHandling	31.6	31.2	−0.38 (−1.2%)
Prime	39.9	39.6	−0.31 (−0.8%)

solution	base	change	delta
FSharp.Compiler.Service repo	731.7	724.8	−6.86 (−0.9%)
Fantomas	291.3	287.2	−4.10 (−1.4%)
ReSharper.FSharp	293.0	290.4	−2.59 (−0.9%)
IcedTasks	90.1	88.3	−1.83 (−2.0%)
Prime	99.2	98.0	−1.23 (−1.2%)
Oxpecker	110.0	109.1	−0.95 (−0.9%)
FsToolkit.ErrorHandling	66.8	66.1	−0.66 (−1.0%)
consoleapp	28.3	28.0	−0.33 (−1.2%)

Correctness

• Diagnostic counts identical in every cell of both sweeps (ReSharper.FSharp 1, this repo 413, Fantomas 20,
  the rest 0).
• Sharing the wrapper shares the binding cache. That is sound because the binding is derived from the
  row and nothing else: two mentions of the same row must resolve to the same entity or the existing
  per-mention resolution would already be inconsistent.
• The cache is per ReaderState, so it never spans two pickled blobs.
• Time: neutral. Measured 2026-08-23 as one rung of a 4-rung cumulative ladder, 5 rounds, 30 samples per
  cell: +0.2% on ReSharper.FSharp and +0.4% on Oxpecker against a +0.6% A/A control.
• Not run: the test suite.

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