Add a segment-aware prefix matching mode to Template #1564

Problem

Template.MODE_STARTS_WITH matches characters, not URI path segments. When routers or applications are mounted under path prefixes on one connector, a route attached to /mcp also matches /mcpx and /mcp-admin/tools:

router.attach("/mcp", mcpRouter);            // Router target => MODE_STARTS_WITH
router.attach("/mcp-admin", adminRouter);
// GET /mcpx            -> mcpRouter, remaining part "x"
// GET /mcp-admin/tools -> mcpRouter (first match), remaining part "-admin/tools"

MODE_BEST_MATCH fixes the /mcp-admin case but not /mcpx. Combining MODE_EQUALS on /mcp with MODE_STARTS_WITH on /mcp/ gets the boundaries right but consumes the leading /, so the mounted router sees tools instead of /tools. Today the only full fix is a TemplateRoute subclass overriding score().

Proposal

Add a third matching mode, Template.MODE_STARTS_WITH_SEGMENT: like MODE_STARTS_WITH, but the match must end on a segment boundary, meaning the end of the string, a following /, ?, ; or #, or a match that itself ends with /. An empty template still matches anything.

router.setRoutingMode(Router.MODE_BEST_MATCH);
router.attach("/mcp", mcpRouter, Template.MODE_STARTS_WITH_SEGMENT);
router.attach("/mcp-admin", adminRouter, Template.MODE_STARTS_WITH_SEGMENT);
router.attach("", rootRouter, Template.MODE_STARTS_WITH_SEGMENT);
// /mcp, /mcp/tools, /mcp?x=1 -> mcpRouter
// /mcp-admin/tools           -> adminRouter
// /mcpx                      -> rootRouter

Existing modes are unchanged, so this is opt-in and non-breaking.

Open question

Should Router.getMatchingMode() later default to this mode for Router and Directory targets (and VirtualHost)? That changes behaviour for existing applications, so it seems better suited to a later major version than to 3.0.

Context

Ikanos (Naftiko) plans to serve its REST, MCP and Skill adapters on one connector, routed by path prefix and by VirtualHost.