feat(def): add cross-file navigation and rename for PSC .def block templates and addresses

Add client and server support to navigate, inspect, reference, and rename block
template and address symbols declared in PSC .def files:

- Client: register language providers for .def (definition, hover, references,
  prepare/provide rename) and send the current document text with each request.
- Server: parse .def files into DefDocument/DefDeclaration/DefReference, expose
  def-specific LSP endpoints (definition/hover/references/prepareRename/rename),
  and integrate .def lookups into existing Tcl hover/definition/references/rename
  flows so Tcl calls jump to .def declarations.
- Tcl server keeps a snapshot API for .def documents (current editor content can
  replace file on request); only declared names in loaded .def files can be
  renamed. Name validation uses DEF_NAME_RE.

Update README and CHANGELOG to document navigation features.

CHANGELOG.md

@@ -5,6 +5,13 @@ Versions correspond to the Git tags of this repository.
 
 ## Unreleased
 
+### Added
+
+- Go to Definition from block template and address arguments in Tcl (`MOM_do_template`, `MOM_force`, `MOM_suppress`, `MOM_ask_address_value`, ...) to the `BLOCK_TEMPLATE`/`ADDRESS` declaration in the PSC `.def` files
+- Hover over block templates shows the template body; hover over addresses shows format, leader, trailer, min/max, and modality
+- Find References and Rename for block templates and addresses across Tcl and `.def` files, including addresses used inside block templates
+- Go to Definition, hover, references, and rename also work inside `.def` files
+
 ### Documentation
 
 - Document DEF block templates, addresses, `BLOCK_LIST`/`ADDR_LIST`, and the formatting changes in the README