[TrimmableTypeMap] Replace fixed value-type container allowlist with scalable NativeAOT rooting

## Context

Follow-up to #12159.

The trimmable typemap currently uses `ValueTypeFactory.SupportedValueTypeFactories` as a fixed allowlist that statically roots the exact NativeAOT shapes needed for:

- managed value-type arrays;
- `JavaList` and `JavaCollection`;
- mixed reference/value `JavaDictionary` shapes;
- the full supported value/value dictionary cross-product.

#12159 extends the list with `DateTime`, `DateTimeOffset`, and `TimeSpan` (plus nullable forms), but arbitrary value types still cannot safely fall back to `MakeGenericType()` under NativeAOT.

## Problem

Growing a framework-owned allowlist does not scale:

- every supported type adds more statically rooted list, collection, and array shapes;
- value/value dictionaries currently root a Cartesian product, so cost grows quadratically;
- applications cannot opt in enums or their own structs;
- other common BCL candidates remain (`decimal`, `Guid`, `DateOnly`, `TimeOnly`, `Half`, unsigned/native-sized integers, `Int128`/`UInt128`, and enums);
- being AOT-rootable is distinct from having built-in JNI primitive marshaling semantics.

## Investigate

- Generate roots from the closed generic container shapes actually used by the application.
- Provide an opt-in registration or source-generation mechanism for additional value types.
- Root only used dictionary key/value pairs instead of the full allowlist cross-product.
- Define behavior for nullable types, enums, and user-defined structs.
- Keep diagnostics explicit when a value-type shape was not rooted.
- Measure NativeAOT build-time and binary-size impact against the fixed allowlist.

## Completion criteria

Produce and validate a design that supports app-specific value types without runtime reflection and without requiring an indefinitely growing, quadratic framework allowlist.

Start with the current ValueTypeFactory.SupportedValueTypeFactories entry point and the design context in #12159. Investigate how NativeAOT roots the closed generic container shapes used by an application, including nullable types, enums, and user-defined structs. Done means a validated design with explicit diagnostics, app-specific opt-in support, and measured build-time and binary-size impact without runtime reflection.

Written by the indexing model from the issue text.