Trimming oversized Message StringBuilder to page size

[the-thing]

Fixes #1137

Changes

• trimming message thread local string builder to max 4096 bytes

Why 4096?

• same as default page size on most operating systems
• Claude gave me the summary

Typical Message Sizes by Type

Message Type	FIX Tag Example	Approx. Size
Heartbeat / Session	MsgType=0	50–150 bytes
New Order Single (D)	Simple FX Spot	200–400 bytes
New Order Single (D)	Fixed Income / complex	400–800 bytes
Execution Report (8)	FX fill	300–600 bytes
Execution Report (8)	Fixed Income fill	500–1,200 bytes
Quote Request (R)	FX RFQ	200–500 bytes
Quote (S)	FX streaming	150–400 bytes
Market Data Snapshot	FI multi-leg	500–2,000+ bytes
Allocation / TradeCaptureReport	Post-trade	800–3,000+ bytes

Note: These are approximate estimates based on FIX 4.x tag=value encoding.
Actual sizes vary by venue, FIX version, and custom tag usage.

[chrjohn]

Thank you @the-thing 👍