Cache decoded string values

[iDavi]

Summary

• add a lazy late-bound cache for escaped string values
• clear the cache whenever a document is reloaded or destroyed
• keep raw-string APIs unchanged

Why

Repeated typed reads of one escaped string repeated the slice, escape scan, and unescape work. The cache retains decoded values only after the first escaped read.

Validation

• inspected the cache lifecycle and repeated-read branches statically
• no VBA runtime or test suite is available in this environment

Closes #5