[Repo Assist] perf: O(1) assembly-name dictionary lookup in convTypeRef (#493)

🤖 *This PR was created by Repo Assist, an automated AI assistant.*

## Summary

Replaces the O(n) `Seq.tryFind` scan in `convTypeRef`'s `bestGuess` step
with O(1) dictionary lookups against the existing assembly-name tables.

## Root Cause / Motivation

`convTypeRef` translates a design-time `Type` to its target-context
equivalent (or vice-versa). For a type `t` that hasn't been seen before,
the previous code did:

```fsharp
let bestGuess =
    asms |> Seq.tryFind(fun a -> a.FullName = t.Assembly.FullName)
    |> Option.bind(fun a -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName a)
```

This is an O(n) scan through all target/source assemblies for every
unique type. With 100–200 reference assemblies and hundreds or thousands
of distinct types in a large generated schema, this can add meaningful
latency during TP warm-up.

`ProvidedTypesContext` already maintains two name-indexed
`ConcurrentDictionary`s:
- `targetAssembliesTable_` keyed by assembly simple name (→
`Choice<Assembly, exn>`)
- `sourceAssembliesTable_` keyed by assembly simple name (→ `Assembly`)

## Fix

Use `t.Assembly.GetName().Name` as a dictionary key for O(1) look-up:

```fsharp
let asmSimpleName = t.Assembly.GetName().Name
let bestGuess =
    if toTgt then
        let table = getTargetAssembliesTable()
        match table.TryGetValue(asmSimpleName) with
        | true, Choice1Of2 asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
        | _ -> None
    else
        let table = getSourceAssembliesTable()
        match table.TryGetValue(asmSimpleName) with
        | true, asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
        | _ -> None
```

The linear scan fallback (`loop`) is fully preserved for the rare case
where the assembly isn't in the table (e.g. type forwarding across
assembly boundaries). The original
`getTargetAssemblies()`/`getSourceAssemblies()` call is also preserved
to populate `asms` for the fallback.

A new `getSourceAssembliesTable()` helper is added (mirrors the existing
`getTargetAssembliesTable()`).

## Trade-offs

- Simple name lookup may theoretically match the wrong assembly if two
assemblies with the same simple name but different versions are present
simultaneously. In practice this does not occur in type provider
reference sets. The fallback loop would still catch it.
- Per-type translation is already memoised in
`typeTableFwd`/`typeTableBwd`, so each unique type is resolved at most
once; the benefit is most visible during initial TP warm-up with large
schemas.

## Test Status

✅ All 126 tests pass (`dotnet test
tests/FSharp.TypeProviders.SDK.Tests.fsproj -c Release`)

> Generated by 🌈 Repo Assist at
[Run](https://github.com/fsprojects/FSharp.TypeProviders.SDK/actions/runs/23826131948).
[Learn
more](https://github.com/githubnext/agentics/blob/main/docs/repo-assist.md).




> Generated by 🌈 Repo Assist at
[{run-started}](https://github.com/fsprojects/FSharp.TypeProviders.SDK/actions/runs/23826131948).
[Learn
more](https://github.com/githubnext/agentics/blob/main/docs/repo-assist.md).
>
> To install this [agentic
workflow](https://github.com/githubnext/agentics/tree/1f672aef974f4246124860fc532f82fe8a93a57e/workflows/repo-assist.md),
run
> ```
> gh aw add
githubnext/agentics@1f672ae
> ```

<!-- gh-aw-agentic-workflow: Repo Assist, engine: copilot, model: auto,
id: 23826131948, workflow_id: repo-assist, run:
https://github.com/fsprojects/FSharp.TypeProviders.SDK/actions/runs/23826131948
-->

<!-- gh-aw-workflow-id: repo-assist -->

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

src/ProvidedTypes.fs

@@ -9197,6 +9197,8 @@ namespace ProviderImplementation.ProvidedTypes
                         ) |> ignore
             sourceAssemblies_
 
+        let getSourceAssembliesTable() = getSourceAssemblies() |> ignore; sourceAssembliesTable_
+
         /// When translating quotations, Expr.Var's are translated to new variable respecting reference equality.
         let varTableFwd = Dictionary<Var, Var>()
         let varTableBwd = Dictionary<Var, Var>()
@@ -9255,10 +9257,22 @@ namespace ProviderImplementation.ProvidedTypes
                 | _ -> 
                 let asms = (if toTgt then getTargetAssemblies() else getSourceAssemblies())
                 let fullName = fixName t.FullName
-
-                let bestGuess = 
-                    asms |> Seq.tryFind(fun a -> a.FullName = t.Assembly.FullName)
-                    |> Option.bind(fun a -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName a)
+                let asmSimpleName = t.Assembly.GetName().Name
+
+                // Use the assembly-name dictionary (O(1)) rather than a sequential scan.
+                // Fall through to None if the assembly isn't in the table, which lets the
+                // linear fallback below handle cross-assembly type forwarding edge cases.
+                let bestGuess =
+                    if toTgt then
+                        let table = getTargetAssembliesTable()
+                        match table.TryGetValue(asmSimpleName) with
+                        | true, Choice1Of2 asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
+                        | _ -> None
+                    else
+                        let table = getSourceAssembliesTable()
+                        match table.TryGetValue(asmSimpleName) with
+                        | true, asm -> tryGetTypeFromAssembly toTgt t.Assembly.FullName fullName asm
+                        | _ -> None
 
                 match bestGuess with
                 | Some (newT, canSave) ->