Fix medium-severity audit findings across all modules

[ConsoleTVs]

Summary

Third and final tier of fixes from the comprehensive security and quality audit of all Cosmos modules. This PR addresses all 42 medium-severity findings (M1–M42) across every module and subpackage.

Contract (M1–M3, M36)

• QueryOr: parse query string once instead of per-call to reduce allocations
• HTMLTemplate: buffer template execution to prevent partial response writes on error
• Add Rehashable interface for hash algorithm migration support

Router (M4–M6)

• Reject patterns containing .. segments to prevent path traversal
• Panic on empty method in Handle() to catch misuse early
• Add concurrency safety documentation for router registration

Problem (M7–M9)

• Ensure deterministic JSON field ordering (type, status, title, detail, instance, extensions)
• Support */* wildcard in Accept header content negotiation
• Fix media type matching to respect slash boundaries

Framework Core (M10–M12)

• Pass request context (not background) to hooks in handleError
• Change DefaultServerOptions to function to prevent global state mutation
• Add panic recovery in Before* hook callbacks on ResponseWriter

Middleware (M13–M18)

• CORS: add Vary headers for cacheability, handle preflight consistently
• Logger: log URL path only to prevent token leakage in query strings
• Rate limiter: respect context cancellation for graceful shutdown
• Recover: capture stack traces for debugging
• Secure headers: remove deprecated X-XSS-Protection header

Session (M19–M22)

• Check MaxLifetime during load to reject expired sessions
• Set sensible MaxLifetime default (24h) when zero value
• Fix Secure cookie flag to use r.TLS instead of request scheme
• Use context.WithoutCancel for session persistence after response

Cache (M23–M27)

• Remove unnecessary inner mutex from entry struct (outer map lock suffices)
• Add documentation warning about reference poisoning with pointer values
• Discriminate cache miss vs computation error in Remember
• Add Redis increment overflow documentation

Crypto (M28–M32)

• Add ErrEncrypterClosed sentinel error for use-after-close detection
• Make Close() idempotent by checking for nil AEAD
• Use clear() builtin for key zeroing instead of manual loop
• Add runtime.KeepAlive to prevent premature garbage collection
• Document nonce size limits and AAD usage

Hash (M33–M37)

• Export BcryptOptions.Cost field for external configuration
• Document bcrypt 72-byte password truncation behavior
• Use clear() builtin for byte slice zeroing with KeepAlive
• Add NeedsRehash to Argon2 for algorithm migration support
• Document argon2 input mutation of password slice

Database (M38–M40)

• Use db.QueryContext/ExecContext directly instead of Prepare+Execute
• Document connection pool configuration recommendations
• Make sqlTx.Close() a no-op instead of closing underlying connection
• Add comprehensive test suite with 14 tests using sqlite3

Event (M41–M42)

• Extract validateEvent helper to deduplicate validation across all brokers
• Document wildcard event matching semantics for each broker

Breaking Changes

DefaultServerOptions is now a function

What changed: DefaultServerOptions changed from var ServerOptions to func DefaultServerOptions() ServerOptions

Migration:

// Before
opts := framework.DefaultServerOptions
opts.ReadTimeout = 10 * time.Second

// After
opts := framework.DefaultServerOptions()
opts.ReadTimeout = 10 * time.Second

Why: Prevents global state mutation where one caller modifying the variable affects all subsequent callers.

CORS panics on AllowCredentials: true with wildcard origins

What changed: (from high-severity PR) CORS middleware now panics at startup if AllowCredentials: true with wildcard * origin.

BcryptOptions.Cost is now exported

What changed: cost field renamed to Cost for external configuration.

Migration: If constructing BcryptOptions with struct literal, rename cost → Cost.

Why: Allows users to configure bcrypt cost without using option functions.