feat: add proc-macro based `tmpl!` and display block template syntax

This introduces a workspace restructuring with separate library and
proc-macro crates, replacing the old declarative macros with a new proc
macro that supports simplified call syntax, implicit template variable
names, and mixed simple parameters with implementation blocks. Display
blocks (`??? >>> name` / `??? <<<`) provide conditionally included
sections with per-arm control, enabled by inserting a param key. The old
`macro_rules!` macros, deprecated module, and standalone test files are
removed in favor of comprehensive inline tests.

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